In Shakespeare's play, Macbeth, Duncan, the King, as arrived at Macbeth's castle to honor him for his extraordinary service in Scotland's recent war. The witches in Act One, scene three, have promised Macbeth that he will be king. In order to help this prediction along, Macbeth and his wife come up with a plan to murder Duncan while he is visiting them.
At the start of the play, Macbeth is a valiant and dedicated soldier for Scotland. He loves his King, and Duncan not only loves Macbeth, but has rewarded him for his service, and promises to continue to do so. Lady Macbeth desperately wants to be queen and she will do whatever it takes to drive Macbeth to raise his weapon to Duncan. When Macbeth's resolve to commit the murder shows signs of weakening, Lady Macbeth nags and berates Macbeth until he crumbles under the pressure. He says that the only thing that motivates him to go forward is his ambition to be king. However, though he agreed to murder the King, he still has doubts: unjust killing does not come naturally to Macbeth—at least at the play's beginning.
In the soliloquy often called the "If it were done when ’tis done" speech, Macbeth worries about the murder. He says that if the killing of the King could be done without raising suspicions of their actions, he would want it done quickly. If by killing the King their success was guaranteed, they would risk the punishment in the afterlife without concern. (Elizabethans believed it a mortal sin to kill a king.)
However, Macbeth believes that what one "sows" in life, one also "reaps" in life—if he and Lady Macbeth spill innocent blood, it's almost certain they will become victims of their actions in this life. If someone visits murder on others, like putting poison in a cup and giving it to others to drink, that murder (or the poisoned cup) will return to punish the murderer(s).
Macbeth goes on to explain the strong and valid reasons he has NOT to kill Duncan. Foremost in Macbeth's mind is that Duncan trusts him—with his life—in that he comes to visit Macbeth at home. Duncan is Macbeth's guest: it was the worst kind of treachery at that time to kill a guest—someone who you were honor-bound to protect under any circumstance, while under your roof. Second, Duncan is a relative. Third, Macbeth is Duncan's subject and owes his King his allegiance. He should be protecting the King from murder, not planning it himself.
Macbeth praises Duncan, noting that he is an admirable man. He is not a tyrant, unjustly ruling his subjects. He is honest, and he such a good man, that all his virtues...
Will pray like angels, as loud as trumpets, against
The deep damnation of his murder.
And pity (personified here) will look upon Ducan's murder and like the innocent confronted by evil, or the highest angels in heaven, and cry out the truth, which will cause torrents of tears to be shed that will "drown the [sound of] the wind."
It is here that Macbeth discloses his tragic flaw: that personal trait that will bring about his downfall. He says he has no good reasons to kill Duncan. The only thing that pushes him forward is his "vaulting ambition," which is ambition so strong that nothing can stop it. He even admits to knowing that this kind of ambition could lead to his downfall (which is, in this case, an example of foreshadowing). So while Macbeth doesn't have the heart to kill Duncan, his feverish, compulsive ambition will push all of his doubts aside.