Reread scene 1.

The Elizabethan's loved a good scary scene. For them that meant witches, thunder, rain....The mood established is ominous and evil. The witches talk about spells and dark things best not mentioned in polite Elizabethan company. The audience knows that no good can come from this. The tragic irony is set up in this scene; Macbeth is the "something wicked this way comes", even if he does not know it yet!