The Headmaster's Only Mistake

When Harry Potter is receives a letter from Gringotts on his eighteenth birthday he is more than confused. But once there, he is far more than just confused. He feels angry, betrayed, disappointed, but most of all, he feels the need to get revenge on those who wronged him. And, that is exactly what he does.
DISCLAIMER: i don't own Harry Potter, if I did, a bunch of characters would be gay and alive