When Damian was killed Batman went after the Heretic and was planning on killing him. Killing him! Now, he didn't because Heretic was a clone of Damian and Batman realized he was all that was left of his son, but he was planning on killing him all the same. So say if it was Joker instead of Heretic, there would have been nothing stopping Batman from taking his life. The Joker would be dead now and that just completely shits all over Jason Todd. Apparently it's not that Batman believes so firmly in what he stands for that he will never take a life, it's just that Jason Todd didn't mean enough to him.