VENICE, Italy — What is it like to look into the eyes of the men who killed your brother and went unpunished? It’s almost impossible to imagine, but “The Look of Silence” provides a glimpse at an answer. Joshua Oppenheimer’s haunting documentary follows Adi Rukun, whose brother died in mass killings in Indonesia in the 1960s, as he tracks down the perpetrators — seeking not revenge, but understanding. His quest is both heroic and brave. The men behind the violence, in which …