You know, "Cain" was an evil son who betrayed and murdered his brother out of jealousy. I doubt that they'll take this route, but it would actually be really cool if the MC chose this name as a representation of his self-loathing and guilt over his brother's death. Maybe as an older brother who was supposed to look out for the family, he feels responsible. Maybe he feels he needs to give up his morals and become accept sin in order to avenge him.