I'm so deeply sorry. I cannot imagine your depth of pain right now.
While your son was being treated for the disease of addiction (which included his family doing all the right things to try to save his life), he lost his life instead to a random and horrific act of violence. Yes, it is a tragedy of unimaginable proportion. One for which you are in no way responsible, for we are all at the mercy of forces greater than our own, over which we have no control. He was an innocent victim, and so are you.
I wish for you love all around you and others to hold you for as long as it takes.