In Shakespeare's Macbeth, I expect that he is at war with himself during the "dagger scene." As he makes his way to Duncan's rooms, he cannot help but think about Duncan—a man he has served faithfully on the battlefield. Macbeth loves his King, who is also his friend and his cousin. He has tried to stop the chain of events by telling his wife they will go no further with their plans, but she rips him apart, insulting his manhood and badgering him into acquiescence. This speaks to Macbeth's inner-weakness, his ambition (which is his character flaw—his "tragic flaw"), and Lady Macbeth's ability to manipulate this husband who at the start of the play seems so dedicated to her—even as she insults him.
Macbeth is quite aware that what he is about to do will compromise his immortal soul: the Elizabethans believed that killing a King was a mortal sin. Only God can ordain who sits on the throne: not a mere man. (Macbeth addresses this awareness in Act Three, scene one.)
It's safe to assume that Macbeth's mind is beginning to fragment: it may well be the witches that present the image of the dagger that seems to float through the air, leading Macbeth on his way to kill the King. However, when he returns to his wife after the murder, we see that he was not at all prepared for the task he carried out. He is undone: raving and hysterical. He even comes back carrying the bloody murder weapons.
… Go, get some water
And wash this filthy witness from your hand.
Why did you bring these daggers from the place?
They must lie there. Go carry them, and smear
The sleepy grooms with blood.
I'll go no more:
I am afraid to think what I have done;
Look on't again I dare not. (II.ii.59-66)
Killing is not unknown to Macbeth, but cold-blooded murder is. It is in this part of the play that we see the last of the good-hearted man who left the battlefield in Act One, scene three. From this point on— and he admits that he expects to see this change—he will find murder easier each time he commits it. He will even leave Lady Macbeth out of his plans and carry out some of the most barbaric acts one could imagine: like killing Macduff's wife and children.