Sister Helen Prejean was not surprised when Loretta Bourque’s parents averted their eyes and hurried past her on a sidewalk in Baton Rouge, La., in 1984.What caught her off guard was David LeBlanc’s father, who stopped and walked right up to her. LeBlanc and Bourque were teenagers who had been murdered by two men, one whom Prejean was trying to save from the death penalty.“Our only son was killed by those two brothers, and all this time, you’ve been visiting those two brothers and you didn’t …