i never heard the "he killed him at the end" theory. and, i dunno, i kinda like it. obviously ambiguous, obviously hidden, but... it does fit. the entire confrontation, batman is telling the joker [paraphrasing] "i don't want either of us to kill each other, but we're running out of alternatives". in other words, he's already putting out the notion that its not just a chance joker will kill him -- he's acknowledging the reverse.
then the last page is 9 panels. the first three are the joke finale. the second three are the two laughing and sirens. the bottom three is "silence, fade to black" style. but the first of those final three panels clearly shows the two characters standing there - just their feet. but the only sound still to be heard (read) is the sirens. the laughter is cut abruptly in the panel before - not even a complete "H" in a "HA". this might be the more appropriate ending to the story, not just because of the title, but really the first time the origin of the joker is told is also his finale.
batman reaches out. offers one last chance. joker acknowledges the attempt, declines, and they both recognize their fate.
it's not bad!
that elseworldsness aside...
there is another story (black and white? legends of the dark knight? i forget) where batman visits barbara in the hospital in a scene that takes place just after the killing joke. she's pissed at him, having heard from other cops that they found him laughing with the joker right after he paralyzed her. outside of her being wheelchair bound, its one of the few references to that story in regular comics -- and i think the only reference to the laughing.
