It's already known that the character isn't morally sound, and being caught after saying "I'll be right back" adds another layer to the morality factor. The Bible contains many stories where people turned away from God, and horror movies picked up on those stories. The horror movie versions are really extreme versions of these stories, but the underlying theme is the same. A more personal sin is committed with the characters. The saying "I'll be right back" adds arrogance to the characters and there is a personal revenge from the God of the film. Sometimes morally questionable characters live in a horror movie, but the characters who say "I'll be right back" never get to the point.