Closer look at Joker from DC Universe: The Exhibit Tour (x)
I just realized something about the DCEU.
It’s established that The Joker killed Jason years ago and Bruce retired, only coming back when Clark’s existence was announced. And when Bruce came back, he was noticeably a lot more brutal and violent and generally unstable than he had ever been before, showing no regard for the lives of criminals. His arc in the movie and presumably the Justice League will be about trying to be closer to the ideal he once established for himself.
Now, one of The main reasons that Bruce has never killed The Joker is out of fear of what he would become if he did, how much easier it would be for him to keep doing it and The Joker wanted nothing more than to bring him down to that level, to make him break his one-rule, even if it was at the cost of his own life. In many ways, The Joker won. He got exactly what he wanted. He broke Bruce so badly that he gave up the cowl and, when he came back, he had given up his morality, his sense of right and had become violent and cruel. In essence, he had become the thing he’d always fought against which is exactly what The Joker wanted.
The Joker has already shown just how dangerous and evil he is without even appearing on screen yet.