Forgiving isn't necessary when a person is not sorry for what they did, IMO. We have been screwed by someone we were trying to help... that person turned out to be one of worst human beings I have ever known. Am I holding a grudge? Maybe... but I don't dwell on it... in fact this is the first time I've thought of it in quite a while.
That experience was pretty unique for me. It takes a lot to get me upset. If someone offends me I almost always give them the benefit of the doubt and just forget about it.