This scene is riveting indeed. It opens with Lady Macbeth having drugged Duncan's attendants and laid out their daggers for Macbeth to use in Duncan's murder. She waits, knowing that the murder is occurring even as she waits: "He [Macbeth] is about it." The immediacy of the murder adds great drama to the scene.
When Macbeth appears, he is covered with Duncan's blood, irrational, and very close to hysteria as his horror overwhelms him. He relives the murder, the crying out of the attendants, and his own inability to say "Amen" when they declared, "God bless us!":
But wherefore could not I pronounce "Amen"?
I had most need of blessing, and "Amen"
Stuck in my throat.
Macbeth continues to unravel emotionally throughout the scene, as Lady Macbeth takes the bloody daggers to return them herself to Duncan's chamber. Her steely calm provides a stark contrast to her husband's terror. As Macbeth's guilt and hysteria continue to build, the scene is suddenly interrupted by a continuous knocking at a door within. The drama of the scene is thus intensified by the intrusion of the world beyond this moment. Macbeth's deed soon will be discovered, as he knows: "Wake Duncan with thy knocking! I would thou couldst!"
The dramatic intensity of the scene chiefly lies in Shakespeare's presentation of Duncan's murder through a system of mirroring: the 'deed' done, the tension of mind the doing of it brings on. Shakespeare does not depict the act of murder in physical terms; he alternatively chooses the psychological responses of the murderer and his collaborator in crime so that the pitfalls of sensationalism can be avoided.
The scene begins with Lady Macbeth's soliloquy which reveals that she is far from a Medea or a Clytemnestra. She needs a stimulant before she can even enter Duncan's chamber just to lay the daggers ready. The soliloquy shows her nervously sensitive temperament:
It was the owl that shriek'd, the fatal bellman,
Which gives the stern'st good-night."
She has invoked the powers of darkness; she has claimed to possess the most abnormal cruelty. But she cannot draw the dagger herself for Duncan resembles her own father.
As Macbeth enters, he sounds rather frightened, remorseful & distracted. Looking on his bloody hands, Macbeth calls it 'a sorry sight'. Macbeth's guilt and fear find expression in his delirious ravings and frenzied outbursts which verbalise the pangs of his tormented conscience, e.g.
"Methought I heard a voice cry 'Sleep no more!
Macbeth does murder sleep..."
"What hands are here? ha! they pluck out mine eyes!
Will all great Neptune's ocean wash this blood
Clean from my hand? No; this my hand will rather
The multitudinous seas incarnadine,
Making the green one red".
Macbeth focalises the theatre in the soul, and the scene of the protagonist's first crime wonderfully underscores the elements of fear, remorse, self-pity & anguish that constitute the theatre in the soul of Macbeth alongside the extreme nervous tension underlying the hard exterior of Lady Macbeth.