I think very dark thoughts about my friend
What about your friend makes you think these dark thoughts?
I realized his influence on me over the year was terrible for me.
You should avoid a situation where someone has a negative influence on you.   You should avoid having dark thoughts about your friend and instead talk to them about the situation so that you think better of them.
Why should I? He has to pay for what he helped put me through.
I understand feeling angry and filled with thoughts of revenge and the dark places that can take you. But I am begging you to talk to him to clear the air first.