I think vengeance does help... like morgan said... and the fact that sometimes people do get away with things without learning a lesson from them... someone has to teach that lesson, if it's not the law or family or society in general, then who is going to take the initiative to teach? If you've been lied to, and the person hurt you and is without shame for what they have done, it is only natural to want to make that perosn understand they hurt you. Vengeance is one of the tools that we have to do so.