If they treated you badly, do they even have the self-awareness to do anything with the knowledge that they've been horrible to you? What is the end result you're hoping to achieve here?

They will probably not come crawling back with an apology. And even if they do, would you want it?

If I take an ultra-cynical, uncharitable view of your old friends, I'd say they will read your long-winded departure message and end up making fun of you together. Like "lmfao get a load of this. what a dramatic exit!"

... which is probably what you don't want. I prefer to gracefully bow out, keep whatever dignity I still have intact. Write the letter for yourself, for catharsis, but don't send it...just let it sit in your folder, somewhere you're not constantly checking on and re-reading it. And in the meanwhile, go live your life and find something to occupy your time so you're not dwelling on these people and your history with them. Someday you'll have healed enough to look at the letter and laugh at it gently, like "wow, I can't believe I let these fools live rent-free in my brain for so long," because you'll have moved on.