An interesting paper in the snappily titled International Journal of Offender Therapy and Comparative Criminology examines what we know about the psychology of revenge.
It has a fascinating section where it discusses how often people take vengeful actions and whether they actually bring any relief.
It seems that taking revenge is rare, but when it happens, it is not only remarkably unsatisfying but counter-productive in terms of dispelling the desire for retribution.
Empirical research by Crombag, Rassin, and Horselenberg (2003) showed that most people do not actually take revenge but merely have thoughts, feelings, and fantasies about it (see also Crombag, 2003). Most people become reconciled with the offender and many people decide to let bygones be bygones. Some of the people who did take revenge could not explain their reason for doing so…
It should be noted that, in the study of Crombag et al., the group of people who took revenge even after a period of time still struggled with more vengeful feelings than the people who did not take revenge. Although 58% experienced satisfaction and 16% experienced triumph, only 19% reported their vengeful feelings to be completely gone, compared with 40% of the people who did not take revenge.
A 2008 study found that one reason that people who do take revenge find it hard to move on is that taking action keeps them ruminating about the events.
Link to locked paper on the psychology of revenge.
Mind Hacks 
Vengeful thoughts by themselves do nothing to harm their object, but do great harm to the one entertaining those feelings. And this harm is both physical and psychological. The only sane and healthy course of action is to let go all negative thoughts. For they are always delusional.
I completely agree.
The summary posted (and abstracts of linked articles) don’t show that failure to let go of vengeful thoughts result in negative outcomes. They show that people who do not let go of negative thoughts suffer negative outcomes. There may be a clear selection bias in these studies. To draw the inferences suggested by the post, you would have to randomly assign wronged people to conditions in which some are required to take revenge, and others are not (I would also include a condition in which people are allowed to choose whether or not they could take revenge). I’d also want to measure some personality traits to assess some possible moderator variables. Until this study is done, all you can conclude is that people who are vengeful and don’t let go, are vengeful and don’t let go.
I can’t help but wonder how much of the desire for revenge is mostly mistaken thinking about what will bring emotional and physical relief.
Might it be that what people who end up feeling like helpless victims are actually looking to do is something that would provide a means of triumphant action? There are lots of ways to accomplish that which don’t require anything at all to do with perpetrators.
Yes, I also agree with the Crombag, 2003 study that most people do not actually take revenge but merely have thoughts, feelings, and fantasies about it. And, that’s way, most people are normal.
I can’t even think of what would happen if everyone go no taking revenge. I believe revenge is psychological problem. Those who can’t let bygones be bygones must be having serious mental issues.