I just don't buy it...(spoilers!)
I think the movie is great, AWESOME, up until the "twist" where Tom reveals who actually shot Valance.
A lot of people clamor that it was "selfless" and "noble" that Tom shot Valance and let Ranse take the credit for it. But I'm just not buying it. Ranse's whole deal from the start of the movie was he wanted to fight his own battles, win or lose. If Tom really didn't believe Ranse could fend for himself in that shootout scene (which he obviously couldn't), well ok; call out Valance and shoot him. But why hide in the alley and time it so it appears to be Ranse that does the shooting? To preserve Ranse's pride? He knows that's not what Ranse's interpretation would be. Is it to allow the people to admire Ranse for the action? Ranse already established his respectability by standing up to Valance on multiple counts. Backing him up in this by shooting Valance in cold blood before Ranse could get a shot off, and thus preserving Ranse's morality would make a whole lot more sense to accomplish what other people are claiming he realized he was accomplishing (paving the way for a new west, realizing his own place in the past, granting Ranse credibility in the future, etc.). Might Ranse have been angry at first because it would seem Tom "bailed him out"? Probably, but the point was (as per Tom admitting "I can live with it") that Tom was saving Ranse from becoming the very thing he set out to rid the world of. Ranse's character would have come around to understanding that.
I just feel like the "twist" is not an organic way to progress the ideas of the story. I feel it's more of a narrative "trick" to get people to empathize with Tom and a heavy-handed attempt to turn him (and the idea of "the old west") into a tragic figure. The movie insists that "The REAL hero turned out to be Tom all along!" and cries "isn't it sad where he ended up?" But I fail to understand to what extent Tom's actions actually make him heroic. He had noble intentions and purpose, yes, but he went about it in an underhanded, and in many ways a petty manner. I already empathized with Tom's character as it was, I didn't need to be "tricked" into it, and thus I actually didn't admire his character as much at the end, because the movie presented him in a way that his actions did not lend to.
I think the film is still very good, but I felt the twist sort of "cheapened" what otherwise was a completely engaging story.