shakey,
I know what you mean a clean kill is the only way to go, first off if I was going to uses a .22 I would shot from inside the house because the walls would trap the noise, with subsonic ammo you don't have the supersonic "crack" of a rifle its more of a pop about as loud as a bullet hiting the chest of your target. the segmented bullets are the safest because they break up and sometimes don't exit and kills are fast,
Now with that said back to the airgun, what I would do here is test some ammo, first get some news paper and let it soak in water overnight
you will need about a two foot stack (I'm guessing you have shot you pellet gun around the house and its safe) take the water loged paper outside and put it at the range you want to test, next fire you pellets in the paper and see what it does.
This is how I test all my bullets for hunting I have found that what they do in the paper is what they do in living tissue an the depth is the same. I have tested everthing I own from a .22 short to a 30-06. also most dogs an coyotes I have shot in the chest will spin and bite there side trying to get at what's stinging them and then roll over an die. unlike a deer that just runs hope this helps