[Seeing that puts a catch in his throat and a pang in his heart. He swallows before responding.]

You cannot kill him without a plan for it. You kill him and you lose him, if he comes back.

I'd like to know for sure whether or not it was him.

[He couldn't care less if the Joker gets wrongfully accused - but he doesn't want someone else getting off scot-free, unblamed and unpunished.]