If you were working with a partner who was holding onto past hurts and revenge, how would you mentor them?
Bitter hearts are the hardest hearts to cure. (I got this idea from a song by Soul Asylum). All you can do is influence your working partner with your positive outlook. The healing of past hurts and revenge should come naturally in time. Although this is not a good thing to work with that kind of person, give him a chance to change. If he doesn't, then both of you will have to move on, possibly towards different directions.
aka Master Joeda