Susan already answered question 1, so I'd like to add a little insight to question 2: My personal opinion is that evil characters have British accents because a Received Pronunciation British accent is associated with people who are aristocratic and powerful, and the evil characters tend to have all the power over the little guy. A common plot in a movie is that the little guy triumphs over the evil villain. Notice it is very uncommon for the villain to be the powerless one and the "good guy" to be very powerful--it just wouldn't make a good movie. It's not that a British (RP) accent sounds evil, it just enables them to make a class distinction between the characters, that is not possible to do with American accents--as there is no aristocratic accent in the US besides the Boston Brahmin accent, which most Americans can't distinguish from a British accent anyway. Most American accents are regional rather than class based. In sum, to many Americans, the Received Pronunciation connotes power and sounds cold and distant. Therefore, it is a "perfect" accent for villains to adopt. (Notice the villain rarely has a Cockney accent, or a broad New York accent--that would make him sound comical rather than evil).

In the case of Scar in "The Lion King", the actor who voices him is Jeremy Irons, who is British and therefore has a British accent. Curiously, Scar is the brother of Mufasa, who's voiced by an American actor, James Earl Jones. As far as I know, nothing in the movie explains why two brothers, presumably raised together by the same parents, would have such different accents ;-) As to the question of why movies villains so often speak with British accents, I have no idea.

Also the British accent sets a character apart in a dramatic fashion. Americans are fascinated by the British accent and can read all sorts of things into it. But I think Brian is right, it has the sound of class snobbery in it.