What's Up With the Ending?
A bunch of people are dead and Will Munny rides off into a rainstorm. Next question!
Okay, it's a wee bit more complicated—although equally bleak 'n' grim—than that.
Everybody who watches Unforgiven is glad that Will comes back and gets his revenge. Little Bill is a bad man, a complete psychopath, and he has no business wearing a badge. Besides, our gut tells us Ned Logan was never as bad as Will Munny, and he didn't deserve to go out the way he did. The final duel between two experienced gun fighters, one of them out for revenge, is about as textbook Western as it gets. After the showdown, Will leaves, never to be heard from again (the departure of the lone gunslinger is also a typical Western convention).
But, because this is Unforgiven, things are made complicated.
First, Will has definitely dishonored his dead wife (Claudia), who once upon a time cured him of his "wickedness," by going back to his old ways. Will claims numerous times to have "changed," but the coldness of his revenge suggests otherwise. Unforgiven leaves us feeling that Will really is just a cold-blooded killer. He's the only guy in the film that seems to have zero problem with violence.
Furthermore, the little postscript refuses to assert with any certainty what happened to Will after the events of the film. While it speculates on the success he may or may not have had, the point is to deny us the "happily ever after" we crave. The point is to not make Will a hero with some kind of prosperous career after the events of Big Whiskey. Will does what he does, and it's best that we know as little about him as possible. That's the message.