Holding onto anger and resentment is dangerous. Does it pose a danger to the person(s) that you are angry with? No. Unfortunately not. You cannot make the object of your rage and resentment combust and turn into an inferno like the X-Man Cyclops just by giving them the evil eye. You wanna know who suffers? You. Yes thats right. You. Depending on who you are holding all this rage in for, they may not even be aware of your feelings and are going about their daily lives happily doing their own thing.
So what does being pissed off to no end at this person do for you? It destroys you. Literally. It makes you a bitter, hateful, miserable person. People can sense there is something going on with you and it will seem like there is a dark cloud that follows you around. Who wants to be around a person like that?
Well, what does harboring resentment and ill will towards a person that you are close to do to you? It does the same thing to you; it eats you up on the inside. Only this time the person that you have all these negative thoughts and feelings probably KNOW that you feel this way and after a time this begins to totally destroy your relationship(s).
Sometimes its hard letting the past go. Its hard letting things lie in the past. Sometimes its hard to let that destructive anger go, but if you want peace of mind and you don't want to push people away from you, just free yourself from it.