I think that maybe your grief is... more than one grief, if that makes sense? You kind of say that in your post: You're not just grieving his death, you're also grieving what could have been.
I grieved my dreams for my marriage-and-perfect-family before I left AXH. But I think you're not just grieving the life you could have had together, but also the life he could have had free from addiction.
To some extent, your post reminds me of how my extended family reacted when a young person in my family (who was also an addict) committed suicide. There was the grief and the loss, there was the guilt (could I have done more? what did I do that made him not reach out for help?) but there was also anger. How could he be so [stupid, uncaring, unloving to his family]?
What I learned about grief when my father died is that all those "stages of grief" things are largely theoretical constructs that sort-of-kind-of try to approximate what someone might be feeling. In reality, grief is a whirlwind of "I should have" and "I wonder if he knew" and "I wonder if I could have done more/acted differently/changed something."
The day I hear that AXH has died, I will probably come to you for help. Because regardless of the fact that we're not in contact, that he was abusive, that I lived in fear of him for years, there is still such a deep, horrid tragedy when a person chooses a route that takes them to living a hell on earth, and refuses any and all help. When you've loved a person who makes that choice, I don't think you can do anything other than feel whatever you're feeling about that choice, and the outcome. (((hugs)))