This 1972 novel is part of the bad seed/creepy kids genre. The title character, Willamina "Willie" Connolly, a precocious young girl whose unresolved Electra complex drives most of her actions, fully lives up to her sobriquet.
I read this book in middle school and loved it. All these years later, reading about wicked children seems to remain a guilty pleasure. I found a used copy at Amazon and can't wait to reread it.