I don't think Montresor gives you many chances. He takes offense at the slightest insult and holds a grudge for a long time. I guess he might be a sociopath, with his mind inventing things that are not there.
It depends on how he has been wronged. Montressor most wanted to get revenge on Fortunado for what seemed like more than one act. Clearly though, if Montressor was wronged to a certain degree, he is not the type of person who can forgive. He has the psyche of a sociopath -- one who is concerned only for himself and shows no or little emotion to events as extreme as executing a murder. Therefore, it is likely that anyone threatening his personal goals or well-being would be subject to a carefully plotted Montressor revenge.