This whole subject is really beyond any of us here in olam hazeh.
As much as we would like to think that a rasha gamur like hitler y’sh is experiencing unending suffering, we might also ask “WHY? – What would be the point of that?” So that he’ll be a good boy NEXT time? But he won’t have a ‘next time’ – will he? Normally when we punish someone, it is to teach him something and/or to accomplish something. What would be accomplished in this case?
Justice l’shma? Maybe.
As I said, I personally think we’ll have to wait til we reach 120 to get the answer to this question.