Don't feel guilty you did your best, personally I wouldn't bother sending anything. Some people will never change and he sounds like one. See his final words to you as what they are, yet another sign of being in control. He couldn't let you hang up on him, he needed to be the one to end everything.

If you need closure there are two options I can suggest the first is to write a letter then burn it. The other is to attend his funeral. I know those are not the most pleasant options but they are the ones that won't leave you a victim yet again.