(Spoiler warning if you have not yet read 'Batman: the Killing Joke'...)
If you haven't yet read it, 'Batman: The Killing Joke' is one of the best Joker stories I've ever read. It tells the origins of the Joker who was previously a man named Jack and it goes more into the mind of the Joker than any other story thus far. (I haven't had time to read them all yet.) But there is one thing that few others really seem to identify and acknowledge at the end of the story. Batman KILLS the Joker. This ending has been debated for years since the graphic novel was released to the public, but just recently in the past year or two, fellow comic book writer to established author Alan Moore, Grant Morrison gave his opinion of the very open ended story. It's that Batman kills the Joker.
Although Batman is now known to uphold his one rule of never taking a life, in this story, he knows it's inevitable that either the Joker or himself will die in the end. I sincerely agree with Morrison's take on the climax of the story. But I have more than just a opinion. I have factual evidence taken from the very pages of the Graphic Novel.
The story begins with Batman taking a trip to Arkham Asylum and speaking to the Joker about his concerns of the outcome of their fate.
Throughout the story Batman attempts to reason with the Joker and convince the clown that he doesn't have to live in this madness. He CAN turn back to sanity. Batman is even willing to help. Because if the Joker refuses his help he is ultimately continuing the course of versus that will in the end with death.
But sadly and as expected, the Joker knows it's far too late for him and that whatever was left of his sanity was lost to that vat of chemicals all those years ago.
He then is reminded of a joke which he tells to Batman. The joke is a analogy of the situation at hand. Two men who are both not all there. One closer to sanity than the other. Batman is the first man who leaped across the gap between insanity and sanity. The Joker is the second man. The one who is far more comfortable on the side of the insane. Batman offered him a seemingly unthinkable way back to sanity. Thus the beam of light across the gap. The Joker believed it possible. But not for him.
This is the end. For both of them. This is the final confrontation of the Bat and the Clown. The finale to the series of the Joker's schemes and the Batman's vendetta against him.
As the laughter that the joke exudes begins, we witness our hero finally snap and break his one rule. He enters albeit for a brief moment, the Joker's world. A world lived in madness. As Batman reaches for his foes neck we clearly see the laughter. Batman is laughing "hehehehe". The Joker is laughing "hahahaha". And as the panel descends to our characters feet we are given crystal clear visuals. The beam of light from the police car is the beam of light across the gap and as this light becomes shorter and shorter we no longer see "hahahaha", but only "hehehehe". Joker's laughter ceases absolutely. Although the Dark Knights continues on. This is the end of the Joker by the killing hands of Batman.