Macbeth is already the thane of Glamis and is extremely popular within the people of Scotland, besides all this popularity and power Macbeth already have the witches persuade him that he can have much more power and popularity within Scotland if he kills the king.
Lust for power
After hearing that Macbeth will become the new thane of Cawdor, Macbeth starts to think wicked thoughts about maybe he can become the king if he plays out the murder like his guards did it. Lady Macbeth is the person that makes Macbeth kill king Duncan because she is evil and has an uncontrollable lust for power.
Tragedy
Macbeth kills king Duncan and breaks the natural order of the world, the people of Scotland are put in a sad state of mind of unrest, unhappiness and fear because of Macbeth wickedness.