I think it'd be fascinating to meet Hitler and have a candid conversation with him. Find out how much he believed, versus how much was propaganda. How much was expediency and how much was fanaticism/passion/dogmatic compulsion.
Of course the same goes for Stalin and Che and Pol Pot and Mao. Or Pinochet. Or let's just get savage and find out more about the minds of Hannibal and Genghis Khan.
Hell, I'd love to put Washington on the hot seat over his role in the Revolution, his interests in whiskey distilleries, and his support over taxation of frontier moonshiners.
There are things to be learned from the minds of corrupt, bad, evil people of all stripes.