There were many people involved in the death of Duncan, the King of Scotland. However, Macbeth bears the major responsibility for the murder. Macbeth committed the task by his own hand. He understood the significance of the prediction in relation to his own ambitions. Finally, Macbeth was aware of his actions and he accepted them.
Macbeth murdered Duncan. He was the one who stabbed the King and he admits that freely in the play. "I have done the deed" relates Macbeth to his Lady after he completed the objective. (II, ii, l.19) Before the murder he says
"I go and it is done; the bell invites me.
Hear it not, Duncan, for it is a knell
That summons thee to heaven, or to hell." (II, I, l.69-71)
In such he plainly states his intent to murder Duncan and again later on, he mentions in a soliloquy that "To know my deed, ‘twere best not know myself." (II, ii, l.92) Preceding the actual death of Duncan, Macbeth’s ambitions became apparent as the significance of the prediction and actual events emerged.
Being an ambitious man, Macbeth said

"… I have no spur
To prick the sides of my intent, but only
Vaulting ambition which o’erleaps itself
And falls on the other." (I,vii,l.25-28)
In this speech Macbeth broadcasts his immense ambitions which are the only reason he is pursuing the witches prediction. Macbeth, upon hearing the witches speak was startled at their prophecy. Banquo said to him "Good sir, why do you start, and seem to fear / Things that do sound so fair?"(I,iii,l.54-55) Macbeth was startled because of the implications of the forecast. Macbeth had thought before about the very thing that he was now being told was his. He was infatuated with the idea and he lusted after information pertaining to it.
"Stay, you imperfect speakers, tell me more:
By Sinel’s death I know I am thane of Glamis;
But how of Cawdor? The thane of Cawdor lives,
A prosperous gentleman; and to be king
Stands not within the prospect of belief,
No more than to be Cawdor. Say from whence
You owe this strange intelligence? or why
With such prophetic greeting? speak, I charge you." (I,iii,l.73-81) Macbeth began to fluster and ramble on, as if in fear that the truth of his thoughts become clear to those near him. He wanted more information from the witches on how he was to become King, but he feared that he would be considered traitorous and disloyal. Which he was, considering that he was reflecting on the possibility of becoming King by murdering Duncan. "’Would they had stay’d!" shows how much Macbeth longed for the throne. (I,iii,l.85) His ambitions were a sources of this longing and desire. The desire for the kingship which he thought through and finally accepted.
Macbeth understood what he was involving himself in. He had thought much about the possibility of assassinating Duncan so that he could take the royal seat. "This have I thought good to deliver thee" he said in his letter to Lady Macbeth. (I,v,l.10) Actually Macbeth thought so much about the prospect that he changed his mind and grew indecisive. Macbeth did, however settle his mind, "I am settled, and bend up / Each corporal agent to this terrible feat." (I,vii,l.89-90) He was terrified of being found guilty and traitorous, so he hid his intentions which shows his clarity of mind and the fact that Macbeth knew what he was to do was wrong and that it horrified him.
"The prince of Cumberland! That is a step
On which I must fall down, or else o’er-leap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires:
Then eye wink at the hand! yet that be,
Which the eye fears, when it is done, to see." (I,iv,l.55-60)
Macbeth charges the stars to shut their light off and to hide his true intentions from everyone around him, including himself. In doing so, Macbeth comprehended what he was to do and accepted the responsibility of the death of Duncan, the King of Scotland.
Macbeth, the thane of Glamis and the thane of Cawdor, holds the major liability for the demise of Duncan. He performed the homicide by his own hands. He was ambitious and understood the relation of the foretelling. Finally,