Ah, again, in the grand tradition of Gotham villains, this Mirror fellow has a tragic backstory to explain his insanity. Batman villains are always insane, it seems. Are there ever any that just want to make a few thousand, quick? Maybe the henchmen, or something.
Anyway, we learn that Mirror is Jonathan Mills, a federal agent and war hero. His family burned to death in a car crash, though he survived. Now, he is 'righting' that 'wrong' by killing people who 'should' have died, but survived. It's a mercy, he claims.
At the end of the issue, he reveals to Barbara that he has placed a bomb on a train, to kill a man who rides that train, and narrowly escaped death, recently. And also to teach Barbara (presumably that it is no good thing to live when one might have died). He declares: "Through fire and explosives and an end to all miracles. You will learn."