To me Scott was ultimately broken by his divorce with Mary and the death of his brother. It was a knockout one-two punch. I never met him, but even as an onlooker I could just tell that he was never the same person after those events. Definitely a broken man.
He also, after all of that and clearly not sober, allegedly did a two-week rehab, finished out dates with VR, then immediately hooked up with STP. Kind of like VR before they got to work on their album, STP should have waited for him to heal. He probably didn't want to, and perhaps Dean/Rob/Eric had decided that they were no longer going to fight the good fight, so the point is a bit moot, but he shouldn't have just switched gears like that.
Once he decided that heroin was the problem and that he could do the other drugs, it was over.
Honestly, though, I am disappointed in the VR guys, who allegedly all relapsed except for Kushner, who KNEW getting into this thing the problems Scott had, and then just were over it and that was it. Someone like Duff knew better than anyone the battle, yet despite knowing what he was agreeing to, he abandoned helping Scott.
Obviously I do not know the details, so I am just saying this based upon what I know. To be clear, though, the buck stops at Scott. If he basically said he never wanted to get fully sober again, then there was nothing anyone could do, even the VR guys.