All villains exist to serve a function of the plot. Only the great ones are compelling characters.
Quick note: this can apply to plenty of adult fiction as well.
Last night, I had the pleasure of having a discussion with a couple of my fellow authors from Pen Name Publishing. For about a half an hour, we talked about the role of women in young adult (YA) literature, paying particular attention to the question: why are so many female protagonists always in love triangles? Basically: why are so many female protagonists defined by their quest to find love? Why is this such a common thread in young adult writing?