Some would say most things you say in a note would only make them feel worse and guiltier, especially if you blame yourself.
As I said in another topic, mine is just a short 6 line typed text that does not blame anyone, straight to the point. It is worth mentioning that I already talked with them plenty before, so they're aware of everything that needs to be said.
In my opinion if you don't want people to suffer keep the letter as "happy" as it can be, make it less about the bad and more about being peaceful about your long term decision. But that's just me.