Earlier today Grant Morrison shared his interpretation of the conclusion to Alan Moore's THE KILLING JOKE in a podcast with Kevin Smith saying
"That’s what I love about it, is that nobody’s noticed–how many, thirty years ago it was? Batman kills the Joker. That’s why it’s called The Killing Joke. The Joker tells "the killing joke” at the end, Batman reaches out and breaks his neck and that’s why the laughter stops and the light goes out at the end, is because that was their last chance at crossing that bridge. And Alan wrote the ultimate Batman/Joker story–he finished it!"
In response to that the letterer of The Killing Joke, Richard Starkings, explains via Facebook the ending of that legendary story.
"No. They are laughing. Batman is laughing so hard he leans on the Joker for support. The End Brian described it to me that way when this craziness first came up In 1986"