They are the bad guys.. why should I care what their motivations are? They are the bad guys. I don't need to know Palpatine's motivation.. he's the villain. Learning what turned Vader to the dark side nearly ruined him as a character. I don't care what Toth-Amon's motivation is, or Sauron's. They are all villains, presented to me as such by authorial intent and I'm content not to need anything else. They are a foil to either make the morally shady character seem less so, or to provide stark contrast to the legitimately heroic characters who seek to do away with them.
I could understand it if your villain wasn't really a villain so much as just a protagonist who was against the heros. Along this line would be the likes of Cobra Commander, Megatron or say for instance Napoleon Boneparte. They aren't exactly villains even though they fulfill the same role in fiction. They are characters who fulfill the Antagonist role, though they aren't exactly evil. This is what saved Vader from being totally destroyed as a character.. he simply was able to swap from being a Villain to an antagonist.. had they not have had the Emperor to show what an actual villain looked like.. it wouldn't have worked.
I won't say this often, but Terry Goodkind actually has a remarkably apt quotation from his first book Wizard's First Rule, in which the Wizard Zedd explains that most evil people don't do things because they think they are evil, but because they think they are the in the right.. it's only everyone else that thinks they are evil. He could have taken his place among the top fantasy writers if he had been cognizant of the fact that he effectively subverted the trope of farmboy saves the world.. by writing a series where the farm boy becomes an even worse dark lord than the one he destroyed, butchers people who don't agree with him, slaughters enemy soldiers, who though are an obviously oppressed people seem far less evil than his own.. It could have been a great variation on the standard theme, and had that have been authorial intent from the get go.. to establish how a dark lord actually becomes a dark lord.. it would have been great..
The Point I'm trying to make is, some times the bad guys do actually wear black hats. They are really irredeemable, they are really bad, and if thats all they are, a nebulous black hat, or a floating fiery Eyeball.. Anything else is just grasping at straws. If the author wants you to know the villains M.O. he/she will include it in the written work.. otherwise content yourself with what you have and move on. Certainly there is no need for fan fiction to fill in the gaps for you.. if you are so desperate to know.. then I suggest you really didn't understand the author's work to begin with, and certainly aren't respectful of it.
Why is this a difficult concept for people to accept? That there just maybe are in fact legitimately evil people in the world. Personally I don't care what Bin Laden's motivation is, even listening to it is some how lending it validity. Bending an ear to your enemies grievances is tantamount to an admission of culpability, complicity or worse guilt for their wrongdoings.
Even though it was really that Rabid Racoon's fault.. Sometimes, there does comes a time to recognize that little Travis just has to put Ol' Yeller down.