So forgive them so that you will be at peace.
If they had the opportunity to be on a spiritual path or in knowledge, they would not have done what they did. So they did it out of their ignorance. Knowing this, you just forgive them so that your mind will be at peace.
Otherwise if you have grudge against someone all the time then your mind becomes so uptight and it creates toxins. If you are feeling negative, then toxins increase. These toxins cause so many illnesses, including cancer. So, we should always keep our mind free and that is why we forgive everyone so that we can be at peace.