9. Murdered Sarah Essen - No Man's Land
Batman: No Man's Land was an enormous comic book event which ran for almost a whole year and span multiple Bat-books. Focusing on the aftermath of a devastating earthquake in Gotham City, the book pit Batman and the Bat-family against wave after wave of villains, fighting for supremacy in a (pretty much) post-apocalyptic landscape. No Man's Land's biggest legacy, however, will be one of the Joker's most heinous crimes: the cold-blooded murder of Sarah Essen.
Tracking down the Joker to the basement of the GCPD - where he was holding dozens of kidnapped orphans - Detective Sarah Essen was the first at the scene. While she ultimately saved the lives of the Joker's baby hostages, she did so by sacrificing her own life for theirs - shot in the head by the Joker at point-blank range. At Christmas, no less.
Even the Joker didn't seem to find his own joke particularly funny, remaining surprisingly stony-faced throughout. Jim Gordon's own gag got a bigger laugh - shooting the Joker's kneecap once they got him in custody. "I may never walk again!" the Joker crowed. "Like your daughter! Good one Commissioner!"