I’ve touched on my thoughts on Batman’s anti killing policy before- but that being said there is a add for escalation. People go on rampages to avoid losing their drivers license in the heat of the moment when they’re cornered and not thinking. Let alone prison time. If you knew being caught meant death- all the stops come out. You have nothing to lose. What’s more- preemptive force escalates. Kill the other guy before he has a reason to kill you. The drug trade is a good example, complex- but one factor in it is escalation.
You have escalation against law enforcement as one factor- “go big or go home,” and escalation against other criminals. As you see lesser criminals escalating force and crime to compensate for escalation in risk and penalty- “bigger” criminals need to escalate too. When every other hood has killed or killed a law enforcer - it isn’t impressive to say. No one says “oh. Don’t mess with Johnny. He killed a guy. He killed a cop. Etc.” the reply becomes “ok. So have all these guys. Why is Johnny so hard?”
You then need to go further. Be “crazier” as reputation isn’t just prestige, it can be the difference between being a kingpin or a dead body. Fear is the order of the street along with respect. If you’re known for torturing people, killing their families, people are less inclined to mess with you. You have to be scarier than “just killing.” And the more intense the guy you want to be better than- the worse you have to be.
Beyond that though- there’s stretching rules and breaking rules. The meter maid might not give you a ticket for having a tiny bit of bumper in the red, or being 19” from the curb. But if you’re 36” from the curb or your whole car except a little sliver is in the red? That’s probably too far. Batman stretches the rules. He lives on the edge of the law- but is largely tolerated. If he became a murderer, he’d be against the system and not for it. He likely wouldn’t be tolerated anymore.
TLDR: Criminals often need to escalate their killing to keep up. Batman doesn't kill because that'd be just over the line.
*taps forehead*