Macbeth's reaction to his murder of King Duncan seems uncharacteristic for a seasoned warrior. Early in the play, the wounded Sergeant reports to Duncan how Macbeth killed Macdonwald on the battlefield without a second thought.
SERGEANT. Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,
Like valor's minion carved out his passage
Till he faced the slave,
Which ne'er shook hands, nor bade farewell to him,
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements. (1.2.19-25)
Macbeth had serious second thoughts about killing Duncan, but once he decided to go through with it, he seemed fully committed to carrying out the murder.
MACBETH. I am settled, and bend up
Each corporal agent to this terrible feat. (1.7.90-91)
Shakespeare doesn't dramatize Duncan's murder, so the audience has no idea what occurred in Duncan's bedchamber. Did Duncan wake up to see Macbeth standing over him? Did they have a conversation, or did Macbeth simply kill Duncan without a word, the same way he killed Macdonwald? Macbeth doesn't say anything about what happened between Duncan and himself.
For whatever reason, when Macbeth returns to Lady Macbeth, he's totally unnerved. He makes no mention of remorse or guilt. He's too overwhelmed to think clearly, and he refuses to think about what he's done. He can't focus his thoughts. He rambles incoherently about hearing voices, real or imagined. Out of the blue, he asks where Duncan's son, Donalbain, is sleeping. Did he have in mind to kill Donalbain and Duncan's other son, Malcolm, as well? It wasn't part of the plan that the audience heard Macbeth and Lady Macbeth discussing earlier in the evening.
Macbeth is in a seriously disordered state of mind, but Lady Macbeth does her best to try to keep him focused on the matter at hand. She appears dispassionate and self-composed only in contrast to Macbeth's behavior. She makes only one passing remark that reflects her feelings towards the murder.
LADY MACBETH. Had he not resembled
My father as he slept, I had done't. (2.2.15-16)
After Macbeth returns to their rooms, Lady Macbeth doesn't have time to think about the murder or even react to it. She's too busy reacting to Macbeth. Why, for example, does it take so long for Lady Macbeth to realize that Macbeth is still holding the daggers he used to kill Duncan? He's been holding them the entire time that he and Lady Macbeth have been talking.
It isn't until Lady Macbeth tells Macbeth to wash the blood off his hands—thinking that the business of murdering Duncan is done—that she sees the daggers in Macbeth's hands and realizes that the situation is unresolved.
LADY MACBETH. Why did you bring these daggers from the place?
They must lie there. Go carry them, and smear
The sleepy grooms with blood.
MACBETH. I'll go no more:
I am afraid to think what I have done;
Look on't again I dare not. (2.2.61-66)
Macbeth is still refusing to face the reality of the situation or think about what he's done, so Lady Macbeth goes into "damage control mode."
LADY MACBETH. Infirm of purpose!
Give me the daggers. ... If he do bleed,
I'll gild the faces of the grooms withal,
For it must seem their guilt. (2.2.67-72)
At the end of the scene, Macbeth is still in denial and still refuses to think about murdering Duncan.
MACBETH. To know my deed, ’twere best not know myself. (2.2.92)
From that moment until the end of the play, Macbeth mentions Duncan's murder only once—when he's considering killing Banquo—but he expresses no emotion about it, other than to refer to Duncan as "gracious." (3.1.70)
Macbeth's reaction to Duncan's murder was immediate but apparently not lasting. Lady Macbeth's reaction to the murder wasn't as immediate as Macbeth's, but she seems to have carried her feelings about the murder with her through the rest of the play—which might have caused her mental instability and ultimately contributed to her death.
LADY MACBETH. ... Yet who would
have thought the old man to have had so much blood in him? (5.1.34-35)