I'm going to destroy a decent friendship that I have.
Why are you going to destroy a friendship?
He started to get on my nerves lately.
Work out exactly what annoys you about the person who is getting on your nerves and talk to them about it. Good communication can save your friendship in the long run.
I want to ruin him, that's why I need to be his enemy.
You don't need enemies you need good friends. You should talk about what is annoying you and work out your friendship.