From what I've read, Steve Jobs was just too full of himself. He had a curable form of pancreatic cancer, and access to any health care conceivable. Instead of that he went with a fruit diet, acupuncture and a psychic. Apparently his diet was so nuts that when Ashton Kutcher tried replicating it, he ended up in the hospital.

Gates while an asshole in the business sense, from accounts that I've heard is a genuinely extremely intelligent person and has great technical understanding. While medicine is not his field from what I heard he seems to take well considered positions in that regard, and I think it's doubtful he'd sabotage his own chances of recovery in such a way.