To forgive is indeed the best form of self-interest since anger, resentment, and revenge are corrosive of that summum bonum, the greatest good.
To take a life when a life has been lost is revenge, not justice.
Before Nelson Mandela was arrested in 1962, he was an angry, relatively young man. He founded the ANC's military wing. When he was released, he surprised everyone because he was talking about reconciliation and forgiveness and not about revenge.
Forgiveness is abandoning your right to revenge.