What I mean is, afaik, there’s a certain stance a target shooter or a hunter would take for greatest accuracy. Do the mass shooters tend to shoot more or less from the hip, or do they actually use the sights? If from the hip, are “regular” rifles less wieldy that way? Do the pistol grips themselves facilitate the speed or accuracy of the shooting, which is usually done while walking? Are the grips more than just scary looking, and would a percentage of deaths be avoided if they just went away?
I generally consider this a more IMHO than FQ question, but I will answer based on my personal experience with pistol-gripped rifles.
I don’t find it makes for a better sight picture, or noticeably easier to control (slightly less if anything, but that depends on how you’re holding/sighting the gun), BUT for removeable magazine fed rifles, it does make reloading a lot easier. Non pistol grip rifles (even semi auto) require a bit more careful handling to reload (not a LOT, but a bit), although it varies a great deal on the firearm in question.
It’s personal preference so there won’t be a FQ answer. Many “experts” now say the “C-grip” technique is best and that is not for a rifle with a front pistol grip. I personally prefer the front pistol grip and feel the C-grip is uncomfortable. For almost my entire Army career the front pistol grip wasn’t a thing.
I’m thinking the OP is probably not a firearms person and is referring to the rear pistol grip of the AR-15 & AK-47 type rifle versus more traditional rear stocks of, say, the M-1 or M-14.
Rather than the recent fashion for front pistol grips on assault-style rifles.
Nost rifles have the barrel above the height of the top of the stock. The AR-15 series has the barrel inline with the top of the stock. As such, you have to have a pistol grip because you can’t hold it like a traditional stock.
As far as I can tell, the pistol grip only improves the shooter’s comfort because of the improved ergonomics of wrist position. The US squad-level light machine gun of World War II was the Browning Automatic Rifle, and it lacks a pistol grip; still, it was an extermely effective and deadly weapon, and changing it to a pistol grip would have simply made it more comfortable to fire from a prone position.
I think the OP needs a bit of education on what exactly an assault rifle is.
Assault rifles were invented by the Nazis in WWII. Back then, you had rifles and you had pistols. You could take rifle cartridges and make machine guns using them, but those tended to be big and heavy weapons with a massive amount of recoil, so typically a machine gun had a bipod, and you would use it by having one person carrying it into place, then drop down and set up the gun on its bipod, and one guy would shoot it while another guy kept feeding it belts of ammunition. They also had submachine guns, aka machine pistols, which fired pistol rounds. Instead of requiring a two man crew, a machine pistol used magazines instead of belts, so a single person could carry it. The recoil from a pistol round is very light compared to a rifle round, so that single person could just aim and shoot. There was no need to drop the machine pistol into position or fiddle with a bipod or any of that nonsense.
(Note - you did also have things like the BAR, but the “walking fire” concept never worked as well as intended, so we’ll ignore that for now)
Russia started fielding large numbers of submachine guns, and the Germans found out the hard way just how effective those were in battle despite their wimpy pistol rounds. The Russian way of dealing with their enemies having superior weapons was to simply throw more men at the problem (which is what they are still doing in Ukraine today), but Germany didn’t have that many men, so they couldn’t use the same strategy. The German military folks got the idea of a machine-gun-rifle kinda thing. Instead of using a pistol cartridge or a rifle cartridge, it would use something halfway in-between, an “intermediate” cartridge. It would be a rifle, but it would operate more like a machine pistol, in that it would use magazines. This limits the rifle to firing in short bursts since you don’t have huge belts of ammunition (just like a submachine gun) but you also don’t need two people to operate it and you don’t need to constantly feed it belts of ammo. The reduced recoil from the intermediate cartridge meant that a single soldier could fire it in full auto bursts but would still be able to keep the weapon on-target.
And thus the modern assault rifle was born. Think of it as a very beefed up sub-machine gun. But, it is not firing rifle rounds. We tried using more powerful rounds with the M-14, and found that the weapon was too difficult to control. If you fired a burst of automatic fire, the first shot would be on-target, the second one above the target’s head, and the third one even higher. It was just too much recoil for automatic fire. Early M-14s were capable of either semi-auto or full-auto fire, but many of those had the selector switch for full auto removed and later M-14s were built from the factory as semi-auto only.
The M-16 is an assault rifle. The 5.56 is an “intermediate” cartridge. It’s basically a machine gun that is designed to fire short bursts of full auto fire.
The AR-15 is not an assault rifle. It is not a machine gun. It cannot fire full auto. The implication of the OP is that the AR-15 is some sort of super-deadly military weapon, a “people killer”. It isn’t. By rifle standards, the AR-15 is fairly wimpy, because again, it’s not actually firing a full-power rifle round. The 5.56 is an “intermediate” round. In fact, in some states you aren’t allowed to use 5.56 to hunt deer because it’s not powerful enough to reliably kill a deer at typical hunting distances.
Is the AR-15 more deadly than a pistol? Absolutely. Again, it’s an intermediate cartridge, halfway between a pistol cartridge and a rifle cartridge. So it is much more powerful than a pistol, but much less powerful than a rifle.
So why the pistol grip? This is because the AR-15 is basically the M-16 with the full-auto option removed, so you’re basically removing the deadly super-submachine gun capability that makes the M-16 an effective military weapon. But that’s the only real change. Everything else is pretty much identical to the military assault rifle M-16. In order to fire accurately in full auto bursts, the M-16 has the shoulder stock inline with the barrel, so that the recoil forces are straight back against the soldier’s shoulder. This allows the soldier to keep the M-16 on-target while firing full-auto bursts. For semi-auto fire, this doesn’t really do much. The wimpy 5.56 doesn’t have much recoil, so in semi-auto mode you really don’t need to worry about keeping the rifle on-target. There’s not enough recoil from a single shot to make the rifle jump.
Since the stock is inline with the barrel, you can’t hold the rifle like a traditional rifle. You need the pistol grip so that you can hold the rifle comfortably.
By comparison, the Soviet AK-47 does not have the recoil forces inline with the barrel, and as a result, in full-auto mode, the AK-47 tends to climb (up and to the left, IIRC) if you fire it in full auto. It also fires a heavier round, which makes the AK-47 even more difficult to keep on-target in full auto. There’s a reason the Soviets switched to the AK-74.
So basically, the pistol grip and the high stock that is inline with the barrel are features designed to make full-auto fire more controllable. Since the civilian AR-15 isn’t capable of full auto fire, these features are pretty much meaningless. They do not make the weapon any more deadly. It is very easy to keep the AR-15 on-target in single-shot semi-auto fire. The 5.56 just doesn’t have anywhere near enough recoil in single shot mode to make aiming difficult.
If you took off the pistol grip, the AR-15 would just be difficult to use because your hand would be up too high to be comfortable. You also can’t put a downward sloping rifle stock on it because the AR-15’s stock contains the buffer tube. When the AR-15 fires, gas from the barrel is ported back to force the bolt to the rear of the rifle. The buffer tube is a spring and a stopper that the bolt hits, so that it stops more gently instead of just slamming back against the rear of the rifle, which reduces the felt recoil of the rifle. The rear stock slides over the buffer tube.
If you want to give the weapon a more traditional stock, you need to redesign it to get rid of the buffer tube, and at that point you no longer have an AR-15.
This, I think, is critical to help keep the rifle from climbing when you fire it. Easier to keep it on target for the next shot.
10/10 for effort but the OP doesn’t use the phrase “assault rifle” even once.
You may be causing confusion because the pistol grip is the thing in the back and the foregrip is the thing in the front. You can be forgiven for all the tacticool douches who say “I put pistol grips on my AR” but you’re gonna get nitpicked to death by the gun crowd.
As far as the meat of your OP, based on my morbid viewing of mass shooter videos, nobody fires from the hip, nor is firing from the hip related really to the type of grips. People who grew up watching Stallone and Arnold learned to fire from the hip, but people who grew up playing counterstrike didn’t. Firing from the hip died with the 80s.
As mentioned pistol grips on automatic or semiauto personal weapons are supposed to help with controlability in full auto and with ergonomics in different firing positions. Though considering the numbers of people slain with traditional-stocked Garands, 1903s, SMLEs, Kar98s, Mosin-Nagants, PPShs, BARs, etc. throughout the prior century, it’s not like it’s indispensable for serious lethality.
More to the point in the particular case of the AR-15 (*) the mechanism involves the recoil spring buffer extending directly in a straight line with the barrel and bolt well past the back of the receiver (main body of the weapon) in a tube. So you with this one you pretty much needed to have an in-line stock and a pistol grip
(* originally created as AR-15 with various options, then the full select-fire version adopted by the military as the M16, then Colt took the original trademark and rebranded the civilian semi-only version with it.)
The point was that the pistol grip on the AR-15 as well as the high stock inline with the barrel are assault rifle features. Since the AR-15 is not capable of automatic fire, those features are not necessary and do not affect the performance of the rifle. They are only important on the assault rifle version of the weapon (the M-16 and its variants).
The OP also implies that the AR-15 is some sort of super-high powered deadly military rifle. It isn’t. To explain why, you need to know what the difference is between an assault rifle and the AR-15. Hence, the explanation of what an assault rifle actually is, why it fires a “wimpy” cartridge compared to a normal rifle round, and why an intermediate cartridge in a semi-auto only rifle exists. Hence the explanation of what an assault rifle is and what the AR-15 is not.
Although, tbf ISTM the OP was more about the preferences of the shooters and how this affected their performance than about the inherent firepower of the weapons.
So to the OP question: the more notorious mass shooters use “tacticool” style weapons because that is what sells in the mass market and they have bought into the hype.
Where?
Here. The implication (or at least the question) is whether or not the AR-15 being a military style rifle with pistol grips is more deadly because of its design, implying that something in that design causes the rifle to be extremely deadly.
Would it be less deadly if it had a traditional stock is basically implying that older rifles with traditional stocks like the M-14 were less deadly, when in fact the opposite is true. The M-14 is a much deadlier people killer than the AR-15.
Man, you guys type fast, have you got pistol grips on those devices? 
Back in the days when engineers had secretaries, I had a secretary that told me I type amazingly fast considering how wrong I did it. 
“Would an AR-15 without pistol grips be less effective at killing people during a mass shooting” is the factual question. The OP didn’t mention older weapons, military weapons, M-14s, assault weapons, or anything else. That’s all.
I simply wanted to know if pistol grips facilitated the mission of the shooter. Maybe there should be competitions pitting “regular” rifles against AR “style” weapons, to see who can put down the most targets in a period of time.
Just out of curiosity, a guy like Rittenhouse goes into a gun store to buy an AR-15, and they’re all out. What’s his next choice?
He’s a dumbass, clearly, who doesn’t know that the M-14 is a deadlier weapon.
There are competitions like that. They aren’t specifically AR vs. non-AR weapons, but there are competitions where you can bring whatever rifle you want, within various categories.
If your goal is to kill people, then the M-14 is a better choice. It packs a deadlier punch with each shot, and is more than accurate enough for typical shooting distances.
If your goal is to knock down targets, the AR-15 is a better choice. Each shot doesn’t pack as much of a punch, but the rifle is generally more accurate (though modern versions of the M-14 exist which are pretty darn accurate).
To reflect that accuracy isn’t the only factor in real life situations, some competitions require you to knock down larger steel targets instead of just hitting them.
The AR-15 is popular because of its “coolness” factor (military style stock, which I personally don’t find “cool” but a lot of modern shooters apparently do), its ability to be customized, and the fact that you can get very cheap ammo for it (pro tip - if you want affordable ammo, pick the same round as your local military, they make a gizillion rounds at a time and thus get a huge advantage for economies of scale)
If you can’t get an AR-15, nothing else on the market has anywhere near as many customization options available. That more than anything probably drives the AR-15 market.
If you just want a military style weapon, there are a lot of options.
FN SCAR which for civilians is available in 5.56 as well as 7.62 NATO (essentially .308) and is also available in 6.5mm Creedmoor (packs less of a punch than .308 at shorter distances but has better long distance performance).
M1A by Springfield Armory. Essentially this is a modernized semi-auto only version of the M-14 specifically for civilians. It is more accurate than the original M-14 and is available with modern military style stocks. You can get it in .308 or 6.5 Creedmoor.
The Spanish CETME which has modern civilian versions. Due to various gun laws the receiver for civilian CETMEs are made in the U.S. and are semi-auto only. The original CETMEs had an old fashioned wooden stock, but a modern military style is available.
H&K G3 the original rifle upon which the CETME was based. There are numerous modern variants available to civilians.
Just like the AR-15 is a civilian version of the M-16, you can also get civilian variants of the AK-47 and AK-74 assault rifles. The original Soviet assault rifles were designed with more of a focus on making them cheap and easily produced, so they aren’t all that accurate. The Russian philosophy was that infantry grunts weren’t expected to be accurate past a couple hundred yards or so. And if they were accurate past that distance, you didn’t give them an AK. You gave them something like a Dragunov SVD instead. But that doesn’t mean that AKs have to be inaccurate pieces of crap. Modern civilian versions are much more accurate than the original Soviet weapons that they are based on.
If you don’t care about having a fancy plastic modern stock, reconditioned M1 Garands are available, though they tend to be a bit pricy. On the plus side, the .30-06 is a very powerful round. On the negative side, learning how to insert enbloc clips quickly and without having the Garand’s bolt slam forward and catch your thumb (google “garand thumb” for more details, it’s a common thing) does take some practice. Once you get the hang of it then reloading is basically about as quick as changing out a magazine, but it’s not as easy to learn how to do it that quickly. While the Garand is far more deadly than an AR-15, Rittenhouse probably wouldn’t choose it because of its old-fashioned look.
Ruger Mini-14 was designed as a miniature version of the M-14 for use by tank crews, artillery crews, and other folks who didn’t need a full-size infantry rifle. Since then it has been made in numerous cartridge types. If you want to know how an AR-15 without a pistol grip would perform, it probably wouldn’t be too far off from an Mini-14 in 5.56 with a classic style wooden stock. The mini-14 is also available in modern military style stocks. The mini-14 is very popular as a ranch gun. It’s good for taking out rabbits, coyotes, and other things that threaten ranch stock. Even with a military stock it’s not really considered a “cool” gun, partly because it’s considered to be underpowered and partly because most people associate it with the old-fashioned wooden stock. But if you have a boatload of 5.56 ammo available and all of the AR-15 and various AR variants aren’t available, the Mini-14 is an option.
Alternately, if what you are really looking for is a “people killer”, a tactical shotgun is extremely effective at closer ranges (within 50 yards or so). They do much more damage to the human body than most rifles, and even pump-action shotguns can fire almost as fast as semi-auto shotguns. On the downside, they suck at target shooting, and they are much slower to reload than most semi-auto rifles. The U.S. military still uses tactical shotguns for urban combat. If your goal is to quickly clear out a building full of bad guys, a tactical shotgun is a good choice.
There are plenty of other weapons available, but this is off the top of my head and I’m running out of time so that’s my list for now.