Originally posted by Gideon
I asked my English teacher as to whether or not a villain could be considered a main character. She said "Oh, absolutely. Nine times out of ten, they are" and began to cite Hannibal Lector, Freddie Krueger, and finally, she mentioned the Emperor. All came back with the same consensus: a villain can easily be a main character, a main character isn't necessarily someone that we must sympathize or empathize with, and the librarian and my English teacher agreed that Palpatine was a main character in Star Wars (ironically, the professor hasn't seen anything but A New Hope).
No one detested here that a villain cannot be a main character, I can give you many examples of this if you wish. Yet the prequels follow the path of a traditional tragedy and although there is antagonism in the piece, obstacles and hinderances you might say, which lead to our main characters demise; there is no main antagonist. Acts of fate and chance are the obstacles traditionally, along with a character flaw that all culminate in the fall.
But the way you describe Palpatine, it seems you intepret the PT as a good vs. evil story, which fair enough, could be the result of shallow analysis, but I assure you, and as several Lucas statements concur, it is a tragedy.
If you fall back to asking English tutors and teachers, as versed as they well may be in forms of literature, they evidently don't have the expertise in cinema and its idealogies.