Usually when someone online says "why didn't you go kill yourself" it's usually meant as wishful thinking.
I said "intense" back pain. The kind of stuff that no amount of Motrin will fix and which can make you instantly crippled and paralyzed at any moment of the damn day.
Thompson did drugs and booze for recreational purposes mainly. I've read just about everything he's written and I never got the impression these substances were a crutch - it was just something he liked to do. On the many occasions he got injured, it was more an excuse to do more drinking than an actual need.
-- edit --
From news coming out after the fact, this was all planned out. He had started leaving instructions for people, and changing his plans for parts of his estate recently. Not in a way that anyone would piece it together before hand, but in hindsight it's more clear that he had made a decision. Apparently the pain he was in from back surgery and a hip replacement made him want to end it before he started to waste away from this chronic disabilities.