I didn't know that about the original Kane character... All the comics I've read and other iterations have pretty much made it canon though that he will NEVER kill, pretty much fact just like Superman's flying ability came into play later in the comics and are now considered essential for the character and to not have that element in an adaptation would be stupid. The same thing for the killing thing for Batman... just because the early year of Batman in the comics showed him killing, that's not the current canon now. He doesn't kill, plain and simple and that's been the accepted facet of his character for greater than 40+ years.
So in light of that, true an adaptation shouldn't be slavish to the comic or source material but basics have to be achieved. Batman doesn't kill. That's a completely key part of his character now for so long and having him so sadistically kill n Returns with Keaton smiling with that **** eating grin is very much out of character and about as stupid as making a Superman movie that wouldn't have flight a major part of Supes abilities (whoops, another Burton and Jon Peters idea). Keaton looks horribly stupid with that smile much like the idiotic dialogue in Batman Begins or TDK's horrible Bale Batman voice...
I digress however, having Batman quit has me in two minds. Part of me is like Bruce Wayne the comic character wouldn't ever do that, ever but again this is an adaptation. For the story purposes of the movie it works brilliantly as an ending. However I didn't like it very much from the context that it's very out of character for comic book Bruce Wayne to do. There are aspects I like about it and things I hate.