He was an odd duck. Talented, yes, way better singer than he got credit for, but troubled from the get-go.
I have none of his solo works, but everything he ever put out in STP and Velvet Revolver, and it always seemed like he was 95% there. Almost great, but not quite over the line.
I've heard "heroin", and I've heard "died in his sleep", though I suppose they are not mutually exclusive.