Warning: spoilers for Batman/Catwoman #6 are ahead.
It is undeniable that paralyzing Barbara Gordon's Batgirl, and a whole host of other horrible things, there appears to be plenty of reasons for Batman to end Joker once and for all. And yet, he refuses to do it.
Over the years, Batman has tried to justify his refusal to kill his archenemy, but none of these reasons have been satisfying enough until now. In Batman/Catwoman #6, while fighting Phantasm, Andrea Beaumont chastises Batman for failing to kill Joker after all of this time (written by Tom King, art by Clay Mann, colors by Tomeu Morey, letters by Clayton Cowles). In response, Batman flatly says, "No one deserves to die."
This answer is refreshing in its simplicity, because it points to a number of core aspects to Batman's character that are sometimes lost in the commotion of his busier comics. What differentiates it from previous takes is the fact that it does not focus on Batman's individual struggle with morality. Instead, it is part of a larger belief that underpins Batman as a superhero to begin with, as a man who fundamentally sees killing as wrong.
In the past, Batman's decision to not kill Joker has been a reflection of his own moral turbulence that he is barely in control of. For instance, in Batman #650 as part of the Under the Hood storyline from writer Judd Winick, penciler Eric Battle, inker Rodney Ramos, colorist Alex Sinclair, and letterer Pat Brosseau, Batman cites his decision to leave Jason Todd's death unavenged by saying that, "I want him dead–maybe more than I've ever wanted anything. But if I do that, if I allow myself to go down into that place... I'll never come back."
Batman's refusal to kill on the grounds that no one, not even Joker, is deserving of such a fate is indicative of the fact that Bruce Wayne is a good man. And while his past moral struggles have provided compelling drama, approaching his adherence to his "no kill" rule in such plain reminds fans of why it is important for him to have these beliefs in the first place. The Dark Knight is a hero immersed in deeply unheroic conditions, but his commitment to upholding his moral standards on the grounds of a basic belief makes him a character both fans and other superheroes in DC Comics can get behind. This is evident in the fact that Batman does know that there is some good in Joker thanks to Martian Manhunter. No one truly deserves to die, not even Joker, and Batman's investment in this is what makes his crusade in Gotham City all the more significant.
