Answer:
They don't really. The Macbeths are looking the innocent flower while being the serpent under it. There is the appearance that the Macbeths are good people and deserve to be king and queen, but in reality they have committed horrible crimes. (Macbeth commits crimes without involving his wife in both acts.) So, nothing is what it seems: fair is foul and foul is fair. But there is no confusion in the audience's mind about what is good and what is evil--we know even if the people in the play do not.