Since Duncan is coming over to Macbeth's castle, this would be the perfect opportunity for Macbeth to make one of witches' prediction come true. Lady Macbeth is upset that her husband is not as ambitious as she when he tells her of his reservations. She questions his manhood. The plan is to make sure that the servents are drunk before they are placed to guard a sleeping Duncan. When the time is right, Macbeth will enter Duncan's chambers and stab him to death. As soon as the deed is done, Macbeth is to leave the daggers with the servents and smear the kings blood on them to frame them of the crime.
From reading of the play its well known that it was lady Macbeth who instigated her husband to murder king Duncan. When she read the letter received from Macbeth stating about the predictions made by the witches, she was sure that his nature was not ruthless enough-- he's "too full o' th' milk of human kindness” to murder Duncan and assure the completion of the witches' prophesy. He had ambition enough, but lacks the gumption to act on it. She then implored him to hurry home so that she could "pour [her] spirits in [his] ear" in other words, goad him on to the murder he must commit.
When she heard from a messanger that King Duncan was coming their home, she called on the heavenly powers to unsex her, taking all her womanly compassion and filling her with cruelty.
Macbeth considered it a terrible sin to kill his king, his kinsman who trusted and loved him most. He told his wife that he could not do it. But Lady Macbeth taunted him and said that he would be there when the king was dead. She herself would do that. When the King went to sleep, his body guards would be drunk and then killing Duncan the blood stained daggers be smeared in their clothes to accuse them of his murder. This was the plan sorted out to murder King Duncan.