I believe that holding on to negative feelings like anger, bitterness, and hatred can endanger one's health. It's best to let go of these emotions so that they do not consume one's thoughts.
That seems to be the moral of a book I've just finished reading, titled "Every Breath You Take" by Ann Rules. It's a true story about a man who was so obsessed with anger and revenge that he hounded, stalked and finally ordered the killing of his ex-wife and mother of his two oldest children. This man was immensely wealthy and had remarried and started a new family, but he was consumed by hatred for his ex from whom he had been divorced for ten years. Imagine spending ten years of one's life hating someone else! In the end, he lost everything when he was found guilty of murder and sentenced to life imprisonment without the possibility of parole.
Hatred is a waste of human potential. Love is so much more fun.