He still quit on his team. We may have sympathy for his situation, but that doesn't remove what he did.
I may be an alcoholic and waste my money on booze as opposed to paying the light, gas and water bills and providing food for my family, resulting in my wife and children starving and being evicted from our home. I may need medical, psychiatric and psychological help and intervention because I've arrived at a point where I can't help myself. Still, I let my family down.
I think that's the proper perspective because it acknowledges issues beyond one's control but holds him/her accountable at the same time.
David Irving did something in his life that made him turn to drugs, in particular, smoking pot. At that point, he had the ability to choose or not choose. He chose to smoke pot. Whatever occurred beyond that point may have sent his life spiraling out of control to a point where he is no longer in control of his life.
But if he ever wishes to get help or acknowledge the cost of what his addiction now has wrought, he has to take personal responsibility for it.
That's how he or anyone else in a similar situation is EVER going to change.