“Well, what can you say after that?” Sen. Tom Udall asked after the screening of “The Act of Killing” at the Library of Congress’ Mary Pickford Theater. The film by Joshua Oppenheimer, nominated for an Academy Award for Best Documentary Feature, examines the lives of the perpetrators of Indonesia’s mass killings of dissidents in 1965-1966, many of whom still occupy places of power and prestige in the world’s fourth-largest country.