You just couldn't let me go, could you?

says the Joker to Batman in The Dark Knight. Rather than let the Joker fall to his death, Batman nabs him with a cable and hoists him back up. Why?

In Batman Begins, in his final fight with Ra's al Ghul, Batman famously says "I won't kill you. But I don't have to save you." So why did he save the Joker? Is there a difference between the two situations?

And is there a moral difference between killing someone and refusing to save someone? Either way, somebody's dead--and it could have turned out otherwise, if a difference choice had been made. How we answer this more general question has lots of serious repercussions for how we think about all kinds of moral issues.

See here for an editorial on whether Batman should just kill the Joker already.