No one has expected the Joker to accidently kill himself. It was just kind of assumed he would keep on living, keep on tormenting until.. actually, no one was sure. With how many close calls he'd managed To survive it was almost like the clown was immortal.
But one Night, it seemed the Joker had gotten caught up in taunting Batman-- who was having a hard time restraining Jason. He accidentally used his real gun, and not the one that shot the flags that read 'pow' or the like.
Everyone watched as joker accidentally shot himself in the head and slumped to the ground. Shock and mortification on their faces. Jason, however, almost cracked a rib laughing. Not because of the supposed mix up, but because it seemed he was the only one who saw you swap them out last second.
After all, if the Joker pulls the trigger it doesn't count because he technically killed himself and not you. But Jason was starting to give it away because he was insisting on giving you a high five in the batmobile. All despite you claiming ignorance.