The only possible reason I could think of torturing someone like Hitler is to try to make them understand the pain they inflicted upon others. But the thing is - they won't understand that what they did was wrong. They'll never understand. If you torture a bad man, they'll never, ever have the sudden realization of "Hey, I don't like this, maybe it was wrong to do it to others!". All they'll think of is "It is wrong for you to do this to ME". People like Hitler have no empathy for others, so torturing them isn't going to get them to suddenly empathize with their victims. They'll never be sorry for what they did - they'll only be sorry that they're getting punished.
So what's the point? There is none. If they are that bad and they are a threat, kill them and be done with it. Torture also has an effect on the torturer. Even if they "deserve" it, if you torture people long enough you'll start to become someone you don't want to be.