Forgiveness prevents the new bad karma of getting back at someone for perceived wrongs

"When you forgive someone who’s wronged you, it doesn’t erase that person’s karma in having done wrong. This is why some people think that forgiveness has no place in the karmic universe of the Buddha’s teachings, and that it’s incompatible with the practice of what he taught. But that’s not so. Forgiveness may not be able to undo old bad kamma, but it can prevent new bad karma from being done. This is especially true with the bad kamma that in Pali is called vera. Vera is often translated as “hostility,” “animosity,” or “antagonism,” but it’s a particular instance of these attitudes: the vengeful animosity that wants to get back at someone for perceived wrongs. This attitude is what has no place in Buddhist practice. Patience can weaken it, but forgiveness is what clears it out of the way.

The Dhammapada, a popular collection of early Buddhist poems, speaks of vera in two contexts. The first is when someone has injured you, and you’d like to inflict some injury back. The second is when you’ve lost a contest — in the Buddha’s time, this referred primarily to military battles, but now it could be extended to any competition where loss entails harm, whether real or only perceived — and you want to get even.

In both cases, forgiveness is what puts an end to vera. You resolve not to settle the score, even if society grants you the right to do so, because you realize that, from the point of view of karma, the only real score in contests like this consists of more bad karma points for both sides. So, in forgiving the other side, you’re basically promising yourself to forego any opportunity to add to the score. You have no idea how many lifetimes this particular karmic mud fight has been going back and forth, but you do know that the only way to end it is to stop the vera, and if the end doesn’t first start with you, it may never arrive."

~ Ṭhānissaro Bhikkhu "All Winners, No Losers: The Buddha’s Teachings on Animosity & Forgiveness"