Does sitting in the car with the key turned (but not all the way) kill the battery?

Eventually it's going to drain the battery.

How depends on how many lights, how much your radio and other stuff draws, and how good your battery is.

But it should be OK for several hours.

As mentioned, keep the key in the "Accessory" position. That way most of the systems (ECU, fuel pump etc) are off and the battery lasts much longer.

Key is on.
Radio is on.
Radio gets power from the battery.
Slowly the battery is drained.
It will kill the battery if you do this for a long time.
Worst is doing it in the winter, because the colder a battery is, the less energy it can give.

I don't know which car you have, but the standard ignition switch/key sequence is:
ACC - only accessories get power
OFF - key can be inserted/removed
OFF - steering wheel is unlocked
IGN - power to engine & accessories
START - starter cranks the engine

So, your key needs to be in the ACC position, NOT the IGN position.

Some vehicles have the positions in a different order.

It use to be that you would turn the key backwards to listen to the Radio, I have noticed these days that you have to turn it one notch forward these days. Depends on the battery and how old it is, new battery should be good unless you just forget to shut it off and go in. If you are worried, just start in every now and again. Let it run a few minutes.

If the car is on, but the engine isn't running, the battery will drain. How quickly depends on how much power you are using up. The radio will most likely take up a good majority of it, but it might be a couple of hours before it makes a difference. The engine turns something called the alternator, which in lamuns terms is a machine that charges your car battery, if the engine isn't on, the alternator isn't charging the battery

It "drains" the battery. That is slightly different thing than "kills" the battery.

A drained battery will recharge. A killed battery is broken inside and will not charge. Every time a battery is completely discharged there is a risk that it will be killed. You would have to listen to the radio until it turns off for the battery to drain. That could be hours to days if the lights are off.

As in, listening to the radio. Yes. It will drain the battery but it might take all night or longer before the car wouldn't start. I have also had batteries go dead from just the dash lights.

It all depends on the battery. A good battery will still crank the car after several hours of listening to the radio. Also, batteries are not all equal. They have amperage ratings. The higher the rating, the longer it will last, but the more expensive they are.

Anything that uses power will eventually kill the battery.

The radio doesn't draw much power in most vehicles ( unless you're running some aftermarket system with high-powered amps, subwoofers, etc. ) so as long as the battery is in the vehicle is fully charged and in good shape, you can listen to the radio for a few hours without worry.

Back in the day, there used to be 2 issues, cars used to have tube radios that used more power, and if the ignition points were closed instead of open, it could melt the ignition coil that makes the spark plugs spark (I have seen that happen). Although, the ignition switch usually had an "Accessory" position, so you could turn on the radio without turning on the ignition.

But unless you have some huge added amplifier, normal solid state car radios use little power. And the engine computer would know not to try to make sparks with the engine not running. So I imagine that you could play the radio for many hours without any issues.

Powering the radio will drain the battery. It won't draw as much power as the lights, but given enough time, it would drain the battery. As a rule, it would take several hours to drain the battery to the point the car would not start.