How does Macbeth's reaction to killing Duncan change by the end of scene 3 in act 2? How is this different from his responses immediately after he...

Macbeth has clearly regained his composure by the end of this scene and is intent on putting the blame on someone else for the king's murder. Immediately after he had murdered Duncan he was anxious and clearly traumatised by what he had done, but now he is in charge and speaks of punishing those who had committed this most foul deed.

Macbeth confesses that he had killed Duncan's two guards once he had re-entered the king's chamber. He states:

O, yet I do repent me of my fury,
That I did kill them.

This was clearly an attempt to get rid of any loose ends. With the guards out of the way, he cannot be implicated.

Before, when Lady Macbeth had asked him to return to Duncan's room to return the guards' daggers and smear them with the king'sblood, he refused, saying:

I'll go no more:
I am afraid to think what I have done;
Look on't again I dare not.

Obviously, a different frame of mind. Macbeth defends his actions by asking in beautifully contrived metaphors and similes, how anyone, when seeing their king lying lifeless and steeped in his own blood, could resist avenging his foul muder when confronted by the sight of his assassins lying there, covered in his blood and still in possession of the weapons with which they committed the deed?

The image is of a plotting and scheming murderer, unlike the fearful and anxious killer he was immediately after he had assassinated his liege. Immediately after the murder, Macbeth had told his wife, when looking at his bloodied hands,

This is a sorry sight.

He went on to tell Lady Macbeth that he had heard a voice stating that, 'Glamis hath murdered sleep and therefore Cawdor shall sleep no more.' and that he could not say 'Amen' when he tried to. he was clearly overwrought and could not think straight. His wife had to encouage him. She called him cowardly when he was startled by knocking. Macbeth finally states at the end of this scene:

To know my deed, 'twere best not know myself.

Knocking within

Wake Duncan with thy knocking! I would thou couldst!

Macbeth clearly regrets what he has done, but at the end of scene three, none of this remorse is evident. He has become chillingly pitiless and has already planned to move ahead. 

Let's briefly put on manly readiness,
And meet i' the hall together.

The blubbering, angst-ridden individual of before is replaced by one who is not only in charge of his faculties, but also of the situation.