In Clint Eastwood's 1992 film Unforgiven, why does Deputy Clyde have only one arm?

First, there’s the obvious fact that Clyde is Bill’s right-hand man. He is both the most competent and the most loyal of Bill’s henchmen. Second, the lawmen are the bad guys in the film. So instead of having a one-armed bandit, we have a one-armed lawman to give us a bit of a twist on an old trope. Third, all of the film’s villains are overcompensating for something (male feelings of inadequacy and the ways in which they overcompensate being a major theme of the film). Clyde carries three loaded guns at all times to make up for his inability to quickly reload with only one hand.

This last one is the most important. Despite his insistence that he doesn’t want to die for lack of shooting back, we only see Clyde fire his gun once in the entire movie (and it isn’t even a return shot). All of his overcompensation comes to nothing. He gets shot at the beginning of the final battle, never manages to get a second shot off despite being alive until the end of the fight, and is killed by the only person in the film to actually use three separate guns effectively. Clyde’s efforts to make himself more formidable in battle come to nothing, just as Bill’s attempts at carpentry ultimately amount to nothing (both because he’s a terrible carpenter and because he dies before finishing the house).

Superb answer, @JeSuisRickSpringfield

In reality, many men of the old west were veterans of the Civil War, and many of those men lost appendages during the war, so ol’ Clyde missing an arm would be very realistic.