Why Batman Does Not Kill Villains ?

He sums it up himself.

If I kill, one day someone is going to tell somebody’s kid that a random person gunned down their parents, just like me. Killing a murderer still leaves a murderer in the world, it doesn’t take one away.

Basically, it’s down to his own back story and moral code.

Despite the fact its often a bizarre choice, it’s one of the things I most like about batman, the fact he’s above the desire to take a life as revenge.