I don’t mean electromagnetic radiation in general, I mean:
Would it be theoretically possible to kill someone with low frequency (like below 1 GHz) but high power radio waves?
Sure.
At the end of the day it is energy and if enough is focused on you it can cause damage.
You would not want to camp next to a high powered radio antenna.
Sounds like this attempt at a literature review on RF hazards, and intensities required for same, might be what you’re looking for. At least the sources mentioned in the article: RF SHOCK AND BURN: Radio Frequency Radiation – Technical Note 124
The Guy and Chou papers mentioned in the review look interesting, dealing with VLF hazards at sites like Jim Creek and others.
Even at low frequency, you want to draw a distinction between “Near Field” and “Radiation”. Near field can kill you at DC: it’s Ionizing (although it’s certainly not Ionizing Radiation). “Radio Waves” drop off quite sharply near the antenna. It would require far more energy if you stood further back. It drops much more sharply than the simple r**2 change with distance that you get farther out with radio Transmission.
You guys can’t prove nothing… 
Microwaves, like those used in microwave ovens, are a subset of radio waves. They won’t give you cancer or anything, but they could cook you.
If you use a frequency that’s less absorbed by tissue (mostly water), then it’d take more power to cook someone, but you can always crank the power up higher. The biggest difficulty would be that the target would have to be restrained, because the first stages of killing someone by cooking them would result in them feeling heat, which will cause them to flee the heat source.
Yes, the distinction of microwave ovens is that they are tuned to the natural frequency of the hydrogen-oxygen bond in water, so they get the atoms moving rapidly, very efficiently.
I don’t know what the frequency of radar typically would be - I heard the anecdote that navy sailors about to go on shore leave believed (with some justification) that standing in front of the ship’s radar antenna for a short time would render them temporarily sterile so they could perform their shore leave tasks with less worry. However, the problem was that apparently too many “treatments” left them permanently sterile.
Also a friend of mine in the Canadian armed forces related the story from many years ago about some arrogant SOB mid-level officer who blatantly ignored a tagged out radar unit, removed the tag and turned it on while it was being worked on. The tech had his exposed arm badly damaged and had limited ability to use it for the rest of his life.
There were also the allegations that the Soviets were trying to do some sort of shenanigans with high-power VLF arrays near the Arctic circle - whether it was submarine contact, or weather experiments, or messing up NORAD radar, I never did see a clear answer.
I’ve heard some sick from songs I thought where pretty ill blasting from some dope speakers. But I don’t think any of them have killed anyone.
My dad only told me not to look into microwave comm links (we didn’t have any radar around). A navy friend told me that he’d seen birds fly in front of the radar and drop out of the sky.
In case anyone reads this and gets confused that it refers to the O-H vibrational frequency, water vapor has a rotational spectrum in the microwave region, but in liquid water it’s more complicated:
10.1016/S0022-3093(02)01084-0
https://www.sciencedirect.com/science/article/abs/pii/S0022309302010840
And microwave ovens are specifically designed to not hit any resonance with water. If they did, they’d have poor penetration depth, and only heat the surface of food, like the infrared in a conventional oven does. But with a penetration depth of a few centimeters, you heat the inside and outside at close to the same rate, which is what leads to the faster cooking that microwave ovens are valued for.
I’m a radar technican and work with high power radar and comm arrays. The short answer is yes.
Having said that, in most reasonable instances you’ll feel very uncomfortable and probably sustain non-lethal injuries because, as noted above, it’ll be cooking you, and that is a distinctly unpleasant sensation you want to be away from (That’s roughly how directed energy weapons work). One of our systems has a peak power rating of nearly 2 GW and while that’s only for a very brief period it’s enough to need nearly a mile safety radius while it’s operating.
The person who invented the microwave oven, Percy Spencer, realized that radio waves could heat things when a chocolate bar in his coat pocket melted when he stood in front of an active radar emitter during WWII.
So it’s reasonable to think that very high powered radar emitters might easily disable a bird, or otherwise cook someone too close to one that is powered up.
You may as well confess now, RadioWave. We have our best people working on your case! The very best people!
Top Men. 
Part of training for amateur radio licensing is learning about RF exposure risks. I doubt there are any hams operating on HF (3-30 MHz) who hasn’t caught a spark and an RF burn. It really grabs your attention!
Here’s a nifty RF exposure calculator:
Hintlink
There’s a reason one of the first brands of microwaves was “Radarange”. And it was enough of a concern that they put a LOT of shielding around the box. Helped move my grandparents unit - even with 2 people it was ridiculous heavy, my guess is 60+ pounds.
One warning/urban legend/rumor I heard from a tech is that you’ll go blind before dieing - fluid in the eye is supposed to absorb better than skin so your eyeballs cook first.
RF burns are nasty. With contact burns, the energy starts at the surface of your skin and goes inward, so by the time you say ouch and pull away, there’s a decent chance that you’ve only damaged the surface layers of your skin. Radio waves penetrate into the skin though, so by the time you say ouch, you have already received a deep tissue burn.
The only human that I am aware of being killed by radio waves was a baby in a microwave, which used to be just a meme until someone went and actually did it (the woman received a life sentence for it). Microwaves are 2.4 GHz though so above the 1 GHz cutoff that the OP is looking for.
I do know someone who has a permanent scar from a waveguide on a radio transmitter that wasn’t supposed to be powered on.
I have heard of animals being killed by radar, but the only personal experience I have is from a bird that went through the radome of an F-16 fighter at 500 mph and embedded parts of itself in the antenna and waveguide (and before anyone makes any smart comments it was the F-16 that was going 500 mph, not the bird, well not until after the collision at least). The bird wasn’t killed by radio waves but it did make for an interesting failure analysis report.
Absolutely true. Eyes (and testicles, if you are male) go first. I think it has more to do with those being relatively delicate structures that are also externally accessible than absorption. Other polarized molecules like fats and sugars also absorb radio waves pretty well.