We touched it in a different topic,but what do you consider the worst thing Joker ever did?
Personally I think the murder of Sarah Essen
http://cdn3.denofgeek.us/sites/denofgeekus/files/styles/insert_main_wide_image/public/joker5_0.jpg?itok=-xlLYoy0 (http://cdn3.denofgeek.us/sites/denofgeekus/files/styles/insert_main_wide_image/public/joker5_0.jpg?itok=-xlLYoy0)
and everything in Emperor Joker share the first place.
http://vignette1.wikia.nocookie.net/villains/images/2/25/Joker-china.png/revision/latest?cb=20110305010827
To me, its a tie between killing Sarah Essen and what he did in `The Killing Joke'.
What really sells that as terrible is that Joker doesnt laugh for the first time.