Without forgiveness, you'll be walking around with a lot of baggage. The more people who hurt you and you didn't forgive them, the heavier the baggage.
I get why you wouldn't want to forgive someone who wronged you. Maybe they did some extremely foul shit. They stabbed your kittens, drowned your dogs and fed you your pet squirrel. I'd want that mfer dead as well.
The reason you forgive them is not for their sake. You do it for you. You do it to lighten the tight grip it holds in your heart. You do it to free your mind from reliving traumatic events.