Obviously Batman won't continue after he kills the Joker. Gordon wouldn't allow it, for one, and Bruce wouldn't want to do it any more. This act is the end. That's not explored because of how the book ends, but it's there. Closing the book with the same structure that began it is a powerful effect, but I think of it as more zooming in to Batman's world for one last time and zooming out when it's done. It doesn't explain away the symbolism of the light. We were just given a powerful and memorable example of the light turning off, with very clear implications as to what it means to the hopelessness of the Joker and Batman's struggle, and then we see it happen symbolically. Also, in the beginning, Batman is going to Arkham to offer the Joker (he thinks) a final chance, so maybe this is when the light is switched on, the beginning of his final attempt at ending it without death, and with the Joker's death it is switched off. I don't think any energy is wasted at all. The question of whether or not Batman will kill the Joker drives the story at a fever pitch, and the fact that he possibly does provides an ending so shocking most people who note that possibility reject it and use the space created by the ambiguity to create a more acceptable one for themselves. And yeah, it is a "spectacular failure" when he kills him. It's not a happy ending. The fatal dance has an inevitable unavoidable conclusion. Even if Batman did slap the cuffs on and put the Joker in a police car, after what he learns in this story he knows one day he will have to kill the Joker or let the Joker kill him. Besides, the more depressing failure is that Batman can't save the Joker because, as the Joker recognizes, Batman himself is too broken to fix anyone. I don't think this is Moore's best work by a longshot. Neither the nihilistic end, nor the more positive possibility are really up there with the endings of his more personal work.But I think the death twist certainly fits everything that leads up to the end better.