9. He Killed Commissioner Gordon
In perhaps one of the most controversial changes to the comic books, Zack Snyder's Batman V Superman: Dawn Of Justice will apparently reveal that The Joker murdered Commissioner Jim Gordon in a past battle with Batman. This will come when Clark Kent visits Gotham City and sees a memorial plaque dedicated to the fan-favourite longtime Batman ally. Perhaps this is being done because Christopher Nolan got the relationship between Gordon and Batman so right in The Dark Knight Trilogy and Snyder doesn't want to try and repeat that. Regardless, The Joker killing such a beloved character will establish him as a serious and deadly threat from the very start. How fans will take this remains to be seen though.