The test to determine reasonableness in self-defense

Dear PAO,

My brother was employed as a laborer or "kargador" in a rice market business. One day, while he was physically unloading sacks of rice from their delivery truck, he was boxed by an unprovoked drunk person who ran amok. He continued working but, upon his return, he was boxed again. This time, he dropped the sack of rice he was carrying and punched the aggressor on the nose, causing him to fall and hit a concrete pavement, which caused his death. In convicting my brother of homicide, the trial court explained that he acted in retaliation, and not in self-defense. How can we weigh the gravity of aggression and the reasonableness of the means employed to repel the attack to justify self-defense?