To put the other side of the argument, especially if you're writing in first person, we can all project shadow qualities onto other people and see them as pretty wicked/nasty at times. And when this is happening, it's hard to see their nicer side. Isn't it the same with 'baddies'? If we're seeing them through the eyes of the protagonist, there's no reason why they can't appear all-bad - though, for me, if later in the book they begin to show human qualities, I feel more affected by what they do. In fact, the more all-bad they appear at the beginning, the more affected the reader will be when the 'baddie' shows he's not all bad.
Ooh, I've just argued myself in a circle.
Susiex