Some individuals have a very difficult time forgiving others when they’ve been hurt. This can be a strong motivator in a story and makes for a great “fatal flaw” if your character is otherwise a good person.
Characters who are at peace within themselves are often rather boring. Those who are walking around with a plethora of grudges, unresolved issues, and hurts are far more interesting. These are issues that everyone struggles with from time to time, if they’re a sentient being. Tie it in with the plot and you have the motivation you need.