Essentially what you do when you enact "revenge" is you become an agent of Karma- you deliver the retribution this individual had coming... the caveat here is that YOU now are the one in possession of the karmic debt. Through an act of vengeance (overt or covert)you effectively transfer the debt- now you are the one who will be forced to pay-up at some point down the line-

Have faith in the karmic process- rest assuredly knowing that all things balance-out eventually- The fact that someone has done you harm, and you have fielded it with equanimity (NOT seeking "revenge") places you that much closer to the goal- as your your karmic debt, in some regard, has been paid- thanks to the individual who has taken on your debt- So, in actuality, this person who you perceive as an enemy is a great ally.