Unhealthy 6 regressing to 3 is a possibility, but I don't think 6 fits him very well. He wasn't security oriented (he visited North Korea), nor was he reliable, stable, self-doubting, or stressful (he was very calm about his cancer). He was so immoderate that many mistake him for an ENTP 7w8. Furthermore, I don't think he was psychologically unhealthy. I see him more as a well developed 5 tending towards 8, which made him more outgoing. When unhealthy, he tended towards 7 (drinking, smoking, disorganization and the like). I also see 4 in his love of art, his iconoclasticism (his natural instinct was always to disagree, something I can definitely relate to), and his rather moody and brooding disposition. It is possible I'm projecting, but I do think 5w4 fits him best.
Zarathustra (from Thus Spoke Zarathustra, not the one on this board. Although, you never know...) 5w4