Clinging to the evil acts people have done is unpleasant. The act does damage once, and then clinging to it continues to hurt you until you forgive the person.
But what if the person is evil? All the more reason to forgive them. To say “this person is unforgivably evil” is backwards. The only reason to forgive someone is for being evil. After all, there is no need to forgive someone for being good.
Forgiving does not equal forgetting. When a person does bad things, it is useful data. Useful data should be preserved. Forgiveness isn’t about consequentialism or justice. Those are extrinsic processes. Forgiveness is an intrinsic process. Forgiveness is simply letting go of grievance and accepting that people are what they are.
What if a person’s acts are atrocious? Bad people are like crocodiles. Crocodiles are dangerous. It is a wild crocodiles’s nature to eat people. Pretending a crocodile is safe just puts people in unnecessary danger, which is bad. Forgetting that crocodiles are dangerous puts people in danger too. You shouldn’t allow a crocodile into your bedroom.
Notice that forgiveness doesn’t have anything to do with strategic decisions. Forgiveness does not mean treating dishonest people as if they are honest, or treating violent people as if they are gentle. External actions are useful to the extent that they facilitate this internal process. It is difficult to forgive someone in your heart while you are screaming at him or her.
Accepting that crocodiles are crocodiles does not put anyone in danger. It’s just a form of accurately modeling reality, which protects people from physical danger.
Meanwhile, the crocodile you observe is a crocodile in your umwelt. When you resent it, you are resenting a part of your own umwelt, which drives disassociation and traps you in samsara.