I always thought it had something to do with the Tower of Babel storyline. http://en.wikipedia.org/wiki/JLA:_Tower_of_Babel
Nah. Tower of Babel just further enforces the fact batman does not kill since he only debilitates Superman by supercharging him to beyond controllable instead of just straight our shooting him with a Kryptonite bullet, which would eventually have killed him. His childhood trauma makes him stupidly apposed to killing.
"Don't fight with stupid people. They will drag you down to their level and beat you with experience." - A guy who clearly lost the fight.