Re: If the author wanted a "killing is always wrong" shpiel, he picked the wrong villains.
One possible counter-argument to that is that Joker and Zsasz are also so thoroughly mentally ill that one can't say they're fully aware of their actions and the repercussions, and so don't deserve to be killed just on that reason. Which is just how Joker managed to skate past the death penalty all those times (except the one highlighted on the previous scans_daily).