I personally don't think Killing applies to pokemon the same way it does Humans. I mean once they are in a POkeball they become immortal, so long as they stay IN the ball. Many pokemon have lived for eons longer than any human would have via capture, Lucario, Rayquaza, Mew, Deoxys, Celebi.. this list goes on.
The other thing about killing is it goes against the honor code instilled by Tajiri-san in the comic book. Which in the pokemon could indeed die, if they battled too far. But then even in the anime, even after fainting it doesnt mean dead. Pikachu has fainted many times and minutes afterwards, its back atop Ash's shoulder.
Thats my reasoning, also it would then suck for kids, hey I got my charizard to lvl 50!... -fights Articuno and (dies) No more charizard- Then the kid doesnt buy ANY more pokemon games ever. lol