Joker kidnaps all of Gotham's nappy-clad infants.
When Sarah Essen-Gordon is the first to discover his location, he throws a baby - yep, a real baby - at her, causing her to drop her weapon and catch the falling child.
Which is practically an open invitation for Joker to shoot her in the head, right? Oh. Well, that's the way he takes it. So he does.
As seen in:
No Man's Land
Why so serious:
She dies bleeding and surrounded by helpless toddlers.
As if that wasn't horrendous enough, this is now the second close relation (wife, in Sarah's case) of James Gordon that Joker has attempted waste at gunpoint. (He previously paralyzed Barbara, his daughter, with a bullet through the spine.)