I experience the same thing. It’s like a death without a corpse. The person I loved and was married to is as gone as if he had been hit by a bus. But he’s also still present, except the version of him today is composed of rage, mental illness and addiction (and rapid deterioration). I too find it hard to grieve for what was lost at the same time as having to deal with the nightmare that he is now.
It is really sad to see a life lost to addiction, as you say, and the fact that it saddens you is a completely healthy response.