When Macbeth is stopped by the three witches with Banquo, Macbeth hears about his prophecy from the witches that he would become king. This instantly puts a thought into Macbeth's mind about killing Duncan. Macbeth says " If good, why do I yet to that suggestion whose horrid image doth unfix my hair"(I,III,130). This shows that Macbeth is having a thought about killing Duncan but it makes him horrified. Macbeth is appointed Thane of Cawdor but is hungry for more power. Lady Macbeth reads the letter and says "Thou'dst have, Great Glamis ,that which cries, 'Thou must thou do, if thou have it' "(I,IV,21-24) . This shows Lady Macbeth's greed for the Throne. Lady Macbeth and Macbeth both speak about killing Duncan because of this prophecy. They both become greedy for the throne. They eventually end up killing the king. This shows that out of pure greed Macbeth murdered the king. If Macbeth never heard of the prophecy this would've not