Macbeth believes he needs to kill King Duncan because he sees the king's son, Malcolm, as a threat to the throne. Macbeth has already felt confused about whether he needs to leave the Witches' prophecy in the hands of fate or do some “dark” deeds to help their prophecies along.

Why shouldnt have Macbeth killed Duncan?

Macbeth is torn between his ambition and his conscience. He gives several reasons why he should not kill Duncan: 1) Duncan is his cousin; 2) He is a loyal subject to the King; 3) Duncan is his friend; 4) Duncan has never abused his royal power; and 5) Duncan is a guest in his home.

Why is Duncan Death important in Macbeth?

In killing Duncan, Macbeth goes against the great chain of being. ... He attacks God through killing Duncan; he undermines God's authority on earth, which will lead to God being very angry, and eternal damnation for Macbeth.