There’s a scene in Halloween (2018) where Michael Myers is following someone while holding a knife and he flips the knife around in his hand.
If you had to kill someone with a knife that was capable of fighting back would it be better to hold the knife like you were cutting a steak with your thumb and index finger against the handle or like you were holding a hammer with your hand completely wrapped around the handle and the blade pointed down?
Or is some other method best?
Depends enormously of the form of the blade, length, your own style of fight (or lack of), the opponent’s style and armament,…
Need answer fast? 
The best way to wield a knife in a ‘fight’ is so you won’t stab yourself or fall upon it while you run away as fast as possible. I’m not being facetious; I spent three months training in edged weapon combat with a former Singapore Police Force officer and instructor who specialized in training edged weapons and had numerous visible scars to attest to his experience. One of the first things he said to me was, “In a real conflict with knives, one guy goes to the emergency room and the other one to the morgue.” Your goal in a knife conflict (not a ‘fight’) is to get in close and disarm and disable your opponent as quickly as possible, which ensures that you are going to be in slashing distance of your opponent. If you are quick enough and not just a little lucky, maybe you can get control of the knife hand or take his legs out from under him, but the odds are good you are going to at least get slashed on the arms, torso, or head while doing this. If someone is wielding two knives, it is almost impossible to disarm and disable them without a standoff or ranged weapon.
As far as the grip it depends on the weapon, technique, and training of the user. The reverse or “icepick” grip with the blade edge facing out from the arm is used primarily with a stabbing weapon like a dagger or smaller knife, or a karambit; this reduces the reach but also makes it more difficult to grapple the knife arm or knock the knife out of the hand. It does require getting close and to the inside. The technique I was taught was to get inside with a broad slash followed by a stab or double stab. I’ve noticed in recent years that a grip with the blade edge facing inward is now favored by some people on the YouTube, under the theory that you can trap an opponents arm with the blade and slash with it; this directly contradicts my training, which is to only trap with the free arm and only to control the weapon or hold your opponent in a more favorable position for strike.
The saber grip (blade out in front and facing down, thumb against the guard or wrapped around the bolster), is useful for larger single-edged knife where it may be used more for slashing than stabbing. The Filipino grip is similar but with the thumb up on the spine to allow greater precision, usually with smaller knives, and is generally used with a rapid stabbing technique.
The punch grip is used with T-handled ‘punch dagger’ or sometimes with small knives like a Spyderco Delica, where the knife is held in a fist and used in jabbing fashion. This doesn’t have a lot of grace but is quite effective if you can get an opening. The T-handled dagger also facilitates retaining the blade after stabbing; something often not appreciated or seen in tv and movie portrayals of knife work is that a blade that is impaled deeply into the chest will often get stuck on bone or by suction and be difficult to extract, leaving you without a weapon and temporarily sealing the wound, preventing the opponent from bleeding severely.
I’m sure you can find dozens of not hundreds of variations on these from people with varying degrees of experience and bullshitititude but they are the basic forms for holding a fighting knife. You often see people on film flipping their knife around as described in the o.p. because it looks impressive (to untrained viewers) but is incredibly dumb, as is throwing the knife at an opponent or waving it around within grappling range of an opponent as some kind of posturing threat. I have been in exactly one ‘knife fight’ in my life when someone took offense at my talking to a woman, pulled out a balisong, and tried to intimidate me by flipping it around with a great show; he managed to flip the blade closed on his fingers and dropped the knife, and I took the opportunity to exit the premises, stopping only to put my glass down at the front door.
Stranger
I watched a show recently and in one scene a character said something like “see if your opponent is holding the knife the wrong way.” As stranger says, there are different grips, but no inherently wrong/ grip. One small factor many weapons are useful because they extend your range. A knife held “ice pick” style gives up a little range.
What’s the best way to wield a knife in a fight?
As a bayonet.
I was trained hands wide out, knife with blade on thumb side, and something around your off hand arm for blocking. I think that is Commando style?
Here is a article:
This may have changed since the 1980’s but Ed Parker, founder of American Kenpo, favored the hilt in the hand with the blade downwards and hidden. Of course, Ed was big on close, multiple strikes.
Legend has it (from my sensei) that Ed and a knife fighter who favored linoleum knives and rumored to have been in multiple real knife fights, had a sparring match. Ed found the dudes weakness, in that his unarmed left arm was extended from the body, and Ed was able to grab the left wrist, extend and take the guy down without being cut. YMMV
Attached to a rifle with a chambered round. And a full magazine.
In a cooking competition.
First, what are the rules?
Always protect your groin.
Being held by a robot. That you are operating from an underground bunker. In another state.
The only winning move is not to play.
That’s not a knife - this is a knife!
I’m right handed. I’d hold the knife in my left hand, the gun in my right, Grenade launcher slung over my left shoulder.
How would you hold your chainsaws, cake guy?
In a similar way, I once asked a friend of mine (who had a black belt in karate) how she well she would do in an unarmed fight.
“I’d run away as fast as possible.”
She had several reasons, but the main one was other attackers joining in.
This method works well even if you don’t actually have a knife:
Heh. They’s too dangerous.
Now back to my cake.