"in fact, i think dragging out his "punishment" at the end weakens the film somewhat.
"perhaps ending after the train scene with the reporters would have been enough, or maybe just enough to make clear that he wouldn't paint again. that would be plenty enough punishment for him.
"the wacky insane homeless guy ending just didn't fit the rest of the picture."
Oh, but it does! It reeks of irony... His oblivious shuffle past his painting (just sold for $10K; a LOT of $$$ back then)... no $$$, no recognition for his art/works, no 'credit' (even for the murder, despite mulitple attempts at redemption via confessions), plus the 'reunion' of his 'wife' and her ex- (which he set up). And who wouldn't? Geezsch, what a schrew!
An amazing piece of film art and wonderful use of noir lighting (albeit for/as 'pulp'), all via a great Director & cast on a low budget (and obvious backlot and studio locales)!
but all the things you mention - money, fame, even credit for the works - didn't hold any interest for him before the ending. it'd be ironic if he cared about those things but since he doesn't taking them away has no impact. of all the main characters in the film, he's the most mentally stable. plus, if anything, he's the true victim and your sympathies are with him. for him to have a mental breakdown and suffer so much for a justified murder (if there can be such a thing) just seems gratuitous to me.
besides, if he can survive years with that awful wife of his without cracking, he should be able to withstand anything!
i agree it's an excellent film - one of the best film bargains i've come across. right up there beside "the stranger" with orson welles from the dollar aisle at target. :)