Honestly?
If it were me, I'd write something that was heartfelt, but ultimately petty and personally embarrassing. It would not give me closure in the end. I don't know whether I would send it and I would be bothered either way. I would fantasize that, after sending it, the person reading it would feel genuine remorse and want to tell me what a bad person they were and have them beg for my forgiveness. But in reality, I'd probably either get a dickish reply, making me feel more enraged, or none at all which would leave me in the same state. I totally understand how and what you feel and it's a pretty crappy feeling.
So I feel you shouldn't do anything. More important is finding closure on your own. So I can only give you advice I probably wouldn't be wise enough to take myself: understand that by being friends with your buddy after all these years is all the 'revenge' and 'closure' you'll ever need or want. You did good and he was wrong. The end.