Yes, certainly the witches, whenever they appear in the play, suggest a very strong image of evil because they appear unnatural. Lightening and thunder introduce them in 1.1, and their even their words portend evil, turning meaning upside down: "Fair is foul, and foul is fair, / Hover through the fog and filthy air" (1.1.11-12). After their first conversation with Macbeth, Banquo says "the earth hath bubbles . . . And these are of them (1.3. 79), suggesting something unnatural. Act 2.3 also uses images of the unnatural, showing nature revolting against itself in protest of the imminent murder of Duncan. Lennox says, "The night has been unruly. . .Lamentings heard i' th' air; strange screams of death,/....Some say, the earth was feverous, and did shake" (55-61).