At the time of the shooting, Hector is in a very bad place. As with many army veterans, he suffers from survivor's guilt and PTSD. He simply hasn't been able to adapt to civilian life properly; he feels that he never really belongs anywhere. His troubled mental state is such that when he shoots a customer at the post office, we immediately assume that he's just snapped.

But, as it turns out, the shooting victim is not just an ordinary customer, but a shady ex-partisan by the name of Rodolfo Berelli. During the war, he'd betrayed many of his fellow partisans, as well as Allied troops, to the Germans. In his desire to escape and get revenge on Peppi for the death of his brother, he came close to killing Hector. Hector never forgot nor forgave Rodolfo's treachery, so, when he catches up with him in America years later, he doesn't hesitate to exact his own revenge.

In the scene in the prologue, it appears that Hector is shooting an innocent customer in the face.  The customer appears at his window asking for a 20 cent stamp.  Hector shoots him in response.  It is not until the epilogue that the reader understands why this has happened.

The customer, you see, is actually Rudolfo, the Italian villager who betrays his peers and the African-American soldiers to the Germans and puts everyone's lives in danger.  Hector is the only one of his group to survive.  Rudolfo also remains protected and untouched at the time. 

The coming together of two men at the end, in the post office, is meant by McBride to be a signal of God's divine will and justice.  Like the arrival at the church during the German seige in St. Anna, the arrival of Rudolfo in Hector's post office is part of a larger plan.  God is giving Hector the chance to avenge his fallen comrades, and Hector seizes that opportunity without pause.