What would you do about your 'friends'?

I have a few friends that I met in college that got i long really well with but recently there's been a lot of drama that we've stopped hanging-out. When we do see each other, which is often, we pretend that everything is ok but are cold to each other. We understand that its ok that we're no longer friends like before. College graduation is in a few days and it may be last time I see them for a while of maybe even ever.

I want to end things in a good note and write them a letter; despite everything I am genuinely proud of them graduating, after all we've been friends since orientation week in 1st year and had good times together, and I also really do wish them the best. And also apologize if I ever hurt them.

On the other hand, when I think of what they did to me and my best friends I still get so mad. I still think that nothing of what they did was ok and that I, writing this letter, will make them think it is. I don't want them to feel that it was ok for them to treat us like they did.

What do I do? Be nice and make peace or end things as they are, anyways we're good not being friends anymore.