I had the urge to write a horrible letter to the man I love.
Why would you do such a thing?
I want him to be afraid of me
It is wrong to write horrible letters to anyone, and especially to someone that you love.  If you succeed in making this man fear you, then he will want to get away from you, which probably isn't what you wanted.
It's just a letter, surely he can't be that hurt by words that I've written? I regret it a little bit already and told him not to read it.
Regardless if it's just the letter or not, what you wrote is going to be hurtful towards him. I know you don't want him to read it, but his curiosity will make him open and read the letter. I recommend giving him a call and tell him not to read it and that you made a mistake.