This isn't really an either/or question. The wonder and tragedy of this character is that she is both. She is a deeply loyal wife, and that leads her into being a monster. She loves Macbeth so much that she knows him and his character well, including that her character is harsher than his, so she must guide him to fulfill his destiny. She isn't pure monster, because she feels guilt so intensely that she scrubs imaginary blood off her hands.