I'm not sure how I would feel about that ending though. Remember Gordon tells Batman he wants Joker brought in by the book. If Gordon didn't cross the line after all the shyt he just had to put up with then surely Batman could keep his composure for the time being and do the same thing.
But it's definitely interesting and your interpretation is certainly plausible. Plus it adds a whole new layer to the title of killing joke.
yeah, i don't necessarily agree with the interpretation that he killed the joker for that reason, i mean gordon gets the worst of it in the book and batman has definitely seen worse (the day his parents died, etc.) i dont think it would be enough to push batman over the line either. but considering the last page by itself makes you think
i think more plausible is that batman is laughing over how insane their whole cycle is: the joker's gonna keep killing more and more people, doin more flagrant shyt every time he gets out, and batman will always keep sending him back to arkham just so he can escape again. both of them know at that point that the cycle will never end, neither the joker nor batman will ever change.