Despicable Me said:
Some people do change, and I give those people a lot of respect for that, even though it doesn't seem to matter to others. Many people in society feel that if you once were a bad person you're always going to be a bad person. But I simply can't accept that. Realization of wrongs done to people is a much, much stronger force than anything like morality or ethics ever could be alone. Society seems to refuse to accept change in people. They distrust it and don't believe it exists.
Turning this on its head, I have applied this to myself as well. I've come to a philosophy which merges mindfulness with acceptance of what I, or whoever it is, has done. I believe that, since the past is locked in and cannot be changed, I or whoever was the baddie am a bad person in that moment. I cannot undo it, I have to accept it as fact, but it doesn't colour my current time, I'm not *bad*, I am bad in that moment in the past, and in that moment I am bad always. Ditto for if it's someone else. In that moment they are bad, and remain bad, but not in this moment. Unless they are.
Some of the family who were the worst to me when I was young, are not at all who they were then. But the evil was done, and I was still hurt. So I can still be angry with them in that moment, but can love this person as they are now.
It's a bit of a caution to guard against being bad, since it will be locked in time. But it also allows me move forward, either away from anger at another person, or away from guilt at myself.
That said, I don't 'forgive and forget'. The one-two emotional punches came pretty constant in my growing up, and I learned to stay safe. I try to forgive if I can, I never forget I just keep it as a data point, and work with it.
In V for Vendetta:
V: What was done to me was monstrous
Evvie: And you became a monster
I think I have that quote right.