Macbeth: Darkness = Evil = Insanity.
Darkness in our society designates evil. Such as a dark place, dark night, or black cat illustrates vile. A diversity of authors uses this method as symbols to describe an evil character or setting. William Shakespeare utilizes the imagery of darkness in the play, "The Tragedy of Macbeth" and symbolizes the "weird sisters" as evil. The witches tempt and torment Macbeth's life by simply planting a seed of evil, making him greedy leading to the murder of Duncan and his downfall.
First of all, the weird sisters are trying to create chaos by prognosticating to Macbeth in order to get him greedy. We learn in Act 1, Scene 1, the witches gather and agree to meet "upon the heath" to greet Macbeth. At the end of the scene, they cry in their cackling voices, "Fair is foul, and foul is fair", which foreshadows the foulness of the plays plot and the tribulation of determining between appearance and reality. As you can see, the witches want to greet him and plan the downfall of Macbeth. In Act 1, Scene 3, Macbeth and Banquo enter as the weird sisters are stirring up their magic while holding hands and dancing around 9 times. Macbeth asks the witches to speak and they foretell the future and tell him he will be "Thane of Cawdor" followed by king of Scotland. Banquo also demands to know his future and they tell Banquo that he will be "lesser than Macbeth and greater" and "not so happy, yet much happier". They also foretell that he shall beget kings. The evil weird sisters poisons Macbeths mind with these prophecies making him gluttonous and bringing out the evil qualities of his soul. When the prediction of the "Thane of Cawdor" comes true, Macbeth then considers the idea of murdering Duncan so he can be king of Scotland. This is the first step on his journey to his failure, as the witches have planned.
Since the witch's plan succeeded, they have stimulated the gluttony in Macbeth, allowing him to make the decision of murdering Duncan.