And as for the voice fluctuation: I think it was done to show that Sidious is not just a man who was scarred by lighting. But much more. A demon taking physical form. Palpatine is more like a possessed vessel for a much greater power, masquerading all this time as a kindly old statesman. An ancient evil bubbling up from the body it now inhabits. Quite literally "The Devil". That's how it came across to me anyway.
Palpatine was obviously fascinated by history as evidenced by the artifacts, statues, etc displayed in his office, not to mention his knowledge of mythology and so forth. Could Palpatine have become possessed at some point by the evil spirit of a Sith Lord while unearthing or studying one of these artifacts? I think it's plausible. Much more plausible than him just being born evil.