From the start Shakespeare introduces evil into the play. The first scene opens with thunder and lightning in a desolate setting. These features are stereotypical devices, because they are closely linked with evil. Enter the three witches. Again, witches are associated with darkness and evil. They are also perceived as un-natural or supernatural beings, which once again brings back the theme of evil. .
The play is always referring to the opposing forces. As shown in Scene 1, act 1, line 4:.
"When the battle's lost and won." This quote suggests opposition in forces because a battle can't be lost AND won. Also in this scene, the quote;.
"Fair is foul and foul is fair". This seems to suggest good against evil, the "fair" or good side, and the "foul" or bad side. I think Shakespeare wanted to open the play with this scene to almost subconsciously set the evil theme for the audience. In this scene Macbeth's name is mentioned, so the audience assume that Macbeth is linked with the witches in some way. .
However, when the audience first sees Macbeth he appears to be brave and heroic in battle, and loyal to King Duncan. This is reflected when the king announces that Macbeth will be made "Thane of Cawdor" due to the previous Holder of the title being put to death, because he was a traitor. Although it is not evident at this time, the King has passed the title from one traitor to another. At the end of the battle, Duncan wins the war. The audience now thinks that Macbeth is a "good guy", so Shakespeare re-introduces the witches in the next scene.
I think Shakespeare does this deliberately to make the audience think that there is still something not quite right in the world. Macbeth's first words in this scene are.
"So foul and fair a day, I have not seen.".
Which is almost a repeat of what the witches were saying in the first scene. Once again this links the witches with Macbeth. Most of the witches speech is in rhyme, almost like a chant, making them appear to have supernatural powers.