I don't think it is that much of an enigma. Just unusual in its portrayal of the hero and villain.
The usual western trappings are there, the town boss, his hired guns, the shootouts, the sidekick, the good girl and the bad girl, and at this level it is all ordinary.
What separates it from the pack are the twists on the characters of the hero and villain. The villain is the stock ruthless town boss with a crooked sheriff and hired gunslingers to keep the local folk under his thumb. The hero is the standard stranger who comes to town on a mission of vengence.
The twist is the private or sexual lives of the two protagonists. Tate is certainly not a conventionally moral man. He keeps a mistress on the side while planning to marry a "respectable" girl. He has had affairs with married women in the past. Yet he comes across as "healthy" and well-balanced. He has been honest and direct with his various women. They went into their affairs with him with eyes wide open. The good girl admits she chased him. Tate likes and seems to understand women. One can imagine Clark Gable or Errol Flynn in this role.
Hero Bart Allison, on the other hand, gives off decidedly "unhealthy" vibes. He has no understanding of his late wife's real character. It is certainly striking that she committed suicide rather than continuing their relationship. What sort of husband could he have been? Rigid, unbending, unforgivingly moralistic, he does not come off as an admirable character, although others in the story seem to admire him.
The last scenes of each of these characters are an interesting reversal of the usual cliches of the time, but fits with what we have observed of them.
Very well explained.