The experience of creating an interesting, multileveled, possibly unique (if there is such a thing) villain is quite simply thrilling.
Some worthy opponent for my hero to show her skills and character by besting.
Some entity that is not cardboard, but a living, breathing person.
My favorite part is that my villain is not evil incarnate or seeking to rule the world or destroy it. I grow so wearing of those types. It is thrilling and entirely satisfying to have created a believable, if not lovable, monster. :)
Has anyone else had as much fun creating their antagonist?