One idea I had for playing villains in a game like it was a Light vs. Grey kind of conflict, where a character could start off as a villain merely because she thinks the means are necessary for a greater goal, in sheer contrast to the heroes' ideals.

Naturally, once a bigger bad is revealed or cleaner means to do good, they could be brought over to the side of the good guys, which also seems to be rather common in Sailor Moon.