The 'Lighter' Side Of The Force
The Emperor has embraced the dark side and the dark side rots him from within. Luke cast aside the dark side and was saved that fate.
Well return of the Jedi came out first so revenge of the sith messed up.
Palpatine's deformed appearance was part of his plan to turn people against the Jedi, so I think he may have let himself become deformed by the fight with Windu. Just as he made it look like Mace was all over him, so Anakin would intervene to 'save' the Chancellor.
In the novelization for RotS, Palpatine states the "The mask is gone now." He says it's a mask himself. Since novelizations of the movie are considered canon, there you go...
Yes I always took it that the senator persona was just the mask for the true underlying character
The Emperor was deformed when he used the power and drained himself and shed the human skin and revealed his true identity.