Bonasera (Salvatore Corsitto) appears at the very beginning of the movie, in the first scene. He's essentially a man who's become disillusioned with the American Dream—although, as he admits, for most of his life, he's done very well in America. As an undertaker, he's managed to prosper, and he has a daughter whom he loves.
But when his daughter refuses to have sex with her boyfriend and another guy, they beat her and injure her horribly. The courts do nothing to really punish them. Bonasera pledges friendship and service to Vito Corleone, convincing the Godfather to inflict pain on the two men who wronged him.
Later, Bonasera does have to repay his debt to Don Corleone—fortunately, not by doing anything immoral. He simply has to make Sonny's bullet-riddled corpse look presentable so that Mama Corleone will be able to look at him in the casket.
Bonasera represents the conflict at the heart of the movie. He seems to be basically a good person, but when circumstances outrage him, he's forced to turn to the Mafia, a system that goes outside the bounds of American democracy and represents an older, almost feudal way of doing things.
The American Dream works well for him… until it doesn't. And when it doesn't, he turns to the more violent methods practiced by the Godfather and his associates for help.