The third part of the witches' prophecy stated that Macbeth would become king after being Thane of Glamis and becoming Thane of Cawdor. After King Duncan makes Macbeth Thane of Cawdor, quite unexpectedly, Macbeth's ambition is ignited and he can think of nothing then except the crown. Macbeth first takes the attitude that if he is to become king, it will happen without his taking any action to promote its happening. However, he soon slips into an active rather than a passive attitude. After Lady Macbeth learns of the prophecy, there is no indecision on her part; Duncan must die in order for Macbeth to gain the throne.
They eventually agree to carry out the plan she has formulated. She will drug the drinks of Duncan's attendants so that he will be unprotected. She will lay out the attendants' swords and daggers for Macbeth to use in killing Duncan while the king sleeps. Then Macbeth will smear Duncan's blood upon his attendants to make them look responsible for the crime.
In the actual commission of the murder, however, the plan fell apart in some details. In his haste and horror, Macbeth carries the murder weapons out of Duncan's chamber and refuses to go back. He is also worried that at least one of Duncan's guards had awakened during the murder. Lady Macbeth returns the weapons herself and completes the staging of the crime. Macbeth later kills the attendants, also, pretending to be overcome with grief upon seeing Duncan's body; his real purpose, of course, was to eliminate any witnesses to his deed.