I doubt it was specifically the painfulness of the particular death that he was trying to escape (by that particular action), as cancer treatment itself is pain

I doubt it was specifically the painfulness of the particular death that he was trying to escape (by that particular action), as cancer treatment itself is painful. It seems to me more likely he was trying to escape preventable death, which is a broad category that could (but in Hitchens' mind probably did not) include aging itself. I think the supposition that he saw death from aging as unpreventable is supported by his lack of explicit advocacy for anti-aging research and by his public dismissal of cryonics. But that's probably because the current evidence is too fuzzy. We don't have the ability to bring back a cryonics patient, and we do know the damage involved in preserving them is non-negligible. If he actually saw reversible human cryopreservation in action, with negligible damage, I doubt he would have abstained from taking a ten or twenty year jaunt to the future to see if they have a better treatment available for his cancer, or any other disease of aging.