Willpower is difficult to measure though. Remorse shows empathy. Empathy for all the pain and suffering you caused by your actions. When you can empathize with the pain and understand what you've done, it's much easier to move onto the path of reform. Even willpower can't always make a person do the right thing if they don't empathize with the effects of their prior actions and understand fully how others feel.Anything can be the cause, but the willpower/resolve is ultimately the driving force behind a man changing his own nature, and the lack of willpower/resolve is also the main reason behind a man's nature being changed.
For example, a drug addict may deeply regret his decision to take up whatever he's hooked on, but without the will to stop, he cannot change.
A person without resolution in his beliefs will change them easily, if belief is what defines a man, one weak of will one shallow in beliefs and thus one who is easily changed by external forces/influence.
IMO, I don't think they're mutually exclusive. You need both in equal parts to truly remake yourself, but first you need to understand or feel remorse before you can even begin to get the willpower to make it right. Otherwise, what are you making right if you don't feel what it was you did wrong?