What was the incident with Chavez and Lupito in the beginning of the novel?

Chavez is the father of Jason, a friend of Antonio's. 
Early in the novel Bless Me, Ultima, Chavez' brother is killed by
Lupito, a World War II veteran suffering from Post Traumatic Stress Disorder.  While
everyone knows Lupito's head isn't right since the war, Chavez cannot let the killing of
his brother go unanswered, and in his own fit of mourning and rage, Chavez is the one
who rounds up a posse to go after Lupito, finally trapping him by the river and
ultimately killing him.

This is an important scene in the
book as it is Tony's first close up view of death, and the first of several in the
story.  He feels sorry for Lupito, and this preares him for his upcoming experiences
with Ultima.