A couple of days ago I ended up on the Letterman site (don't ask; long story) and saw that they had a clip of Richards' appearance (long distance), talking to Letterman and Richards' friend Jerry Seinfeld, who'd convinced Letterman to give the guy a chance to talk and apologize.
I found it very moving, actually. He seems stunned, although entirely rational. The more I listened to him, the more I was thinking of Lenny Bruce, which is a long and similarly affecting story. Bruce was ferociously funny, but his humour arose from personal fury as well as an exceptionally keen and brave political critique. Put the two together and you had explosions -- of genius, but also of self-destruction. Mind you, you could say the same thing of Jonathan Swift, eg (who probably ended up with dementia, or maybe it was tinnitus bad enough to drive him bananas).
I don't mean to make the romantic argument about artistic genius being allied to madness, although there are so many examples. There are counter-examples too -- Tom Lehrer was a near contemporary of Bruce's with a similarly fierce take on American politics and a similarly sharp wit, and yet he did not consume himself.
But watching and listening to Richards, I was starting to make some sense of what he did. He was trying something outrageous in his act and it just wasn't working -- it was a dumb gambit (dumb in the way that a lot of pop music is, actually), and he tried to get out of it by going even further over the top.
At the same time, there seems no question that his comedy comes from a space of some personal anger.
Maybe he self-medicates -- a lot of people do. And I agree with lagatta that that explains nothing to anyone except, maybe, the psychiatrists who have a vested interest in creating angelically mindless categories like "substance abuse."
Anyway. I hope he pulls out of this. It was an interesting segment to watch, and I ended up feeling a lot of respect for Jerry Seinfeld.