Why is Lupito killed?

Basically, Lupito, a veteran, is killed because he murdered the town sheriff, who is also Chavez's brother.
In the book, Chavez relates to Gabriel, Antonio's father, that his brother had been drinking coffee at the bus depot cafe when he was shot by Lupito. To get revenge for his brother's death, Chavez and Gabriel join a posse to hunt down Lupito.
Meanwhile, Antonio decides to follow his father and Chavez to the scene of Lupito's execution. What Antonio sees, however, will forever be imprinted in his memory. Accordingly, a group of men are standing in wait on a bridge. Below them hides the crazed and desperate Lupito. The men have no inkling about Lupito's location, however, until Antonio chances upon the scene. Seeing Lupito so unexpectedly under the bridge gives Antonio a fright, and the latter cries out.
Antonio's scream alerts the men to Lupito's presence, and someone shines a light in the vicinity of his location. Eventually, Lupito lets out several unearthly screams, warning of approaching Japanese soldiers. By now, we understand that the experiences of war have turned Lupito into a tormented dysfunctional version of his former self. In the end, Lupito is shot dead by several of the men in the posse.