Angry, Vengeful, Trying To Calm Down

Okay, so someone very close to me was attacked by our next door neighbor. She didn't tell us for almost a week and when she finally did, we convinced her to take out a restraining order. It went through, but she never pressed charges, mainly because of his police connections and her humiliation. This was back in August and I still find myself enraged. Sometimes at random, my blood just boils and I get so angry. I don't have an outlet for this anger and it honestly scares me. I know I have an anger issue, but I've never felt rage like this and I don't know how to quell it. What can I do?