Nothing happens to them when you forgive them...it happens to you.
Yes bitterness and resentment creates toxic chemicals in your body...the question is, why do you want to create toxic chemicals in your body?
You know when you forgive someone when you think of them or see them and you don't have bitterness and resentment, when your blood pressure doesn't rise and your blood don't boil.
Not forgiving someone is like drinking poison and hoping the other would die....does that make sense?
Forgiveness, Unconditional love, tis the answer.