Mesillas Yesharim says: Hatred and revenge are also difficult to escape. A person is very sensitive to being insulted, and his only comfort from this anguish may be taking revenge, which is to him sweeter than honey. It requires the utmost strength and courage to overcome the natural tendency to hate the one who offended him and not to hold a grudge against him... This may be easy for angels who do not have a nature to hate when offended, but it is extremely difficult for mortals... Yet the DIvine decree is explicit: "Do not take revenge nor bear a grudge." Leviticus 19:17-18 Not only is revenge forbidden, but even when doing a kindness to someone who has injured you, you may not say, "I am more decent than you"

- Mesillas Yesharim ch 11