King Duncan passes the sentence of death on the original Thane of Cawdor after he betrayed him by fighting on the side of his enemies, MacDonwald and Sweno, the king of Norway. Once the enemy has been defeated and Cawdor arrested, Duncan commands:
"... go pronounce his present death'
And with his former title greet Macbeth."
The title "Thane of Cawdor" has been bestowed on Macbeth, and this, in itself, foreshadows Macbeth's own future betrayal and his doom.
When Macbeth and Banquo encounter the witches, the second witch greets Macbeth thus:
"All hail Macbeth, hail to thee, Thane of Cawdor!"
The third witch adds:
"All hail Macbeth, thou shalt be king hereafter!"
Ross and Angus later meet Macbeth and Banquo and Ross informs Macbeth that the title of Cawdor had been bestowed on him. Macbeth remarks during an aside:
"Glamis, and thane of Cawdor!
The greatest is behind!"
Macbeth's response clearly indicates his intent - the greatest hurdle to his ambition has been overcome. The witches' prediction has come true. The fact that he has been given the title "Thane of Cawdor" sets in motion the tragic events which later lead to Duncan's assassination, Macbeth's tyrannical rule and eventually his death.
Ironically, Macbeth is killed for exactly the same crime the original Thane of Cawdor committed: treason. Only this time, Macbeth has committed the ultimate betrayal - murdering his own king and turning against his own kin and kindred.
The original Thane of Cawdor was executed for treason because he fought with the Norwegians against Scotland. His execution foreshadows Macbeth's own death later in the play. Ironically, Macbeth's rise in power begins when he is given the executed thane's title and possessions as a reward for his bravery in battle. Although he fought to defend King Duncan and Scotland then, after he becomes the Thane of Cawdor as the witches had predicted, Macbeth begins to plot his ascension to the throne, and his own downfall ensues.
At the start of the play, the original Thane of Cawdor has already committed treason against Scotland by assisting the King of Norway in battle against King Duncan’s army. If not for Macbeth’s bravery and skill, Norway might have defeated Scotland. When King Duncan learns of this in act 1 scene 3, he orders Ross to go have The Thane of Cawdor executed, then he announces that Macbeth shall be titled the new Thane of Cawdor (in addition to already being Thane of Glamis). However, Duncan is saddened by the betrayal. When he bequeaths the title to Macbeth in act 1 scene 4, he says of the former Thane, “He was a gentleman on whom I built / An absolute trust.” Malcolm reports that the Thane confessed and begged the king’s pardon, dying bravely, with more honor than with which he lived. Duncan responds that you can’t tell what is in a man’s heart by looking at his face, a truly prophetic statement.
In a similar fashion, King Duncan fully trusts Macbeth and honors him with titles and power in act 1 scene 4, telling him, “I have begun to plant thee, and will labor / To make thee full of growing.” Mirroring the Thane of Cawdor, Macbeth then betrays his king, but in an even more heinous way, by murdering him in Macbeth’s own castle. In spite of all the power he gains as the new king, Macbeth's evil behavior becomes his own downfall, just as it did for the original Thane of Cawdor. And like him, Macbeth dies bravely, telling his executioner Macduff, “I will not surrender.../ I will fight to the end.” Yet the end should not surprise us. Knowing the outcome for the Thane of Cawdor from the beginning, we could see Macbeth’s demise looming.
With the execution of the Thane of Cawdor, a title Macbeth is given by King Duncan for his bravery on the battlefield, Macbeth's future actions against the king are foreshadowed. He too will commit a horrific act of treason, killing the king, or regicide.
So the title of Thane of Cawdor inspires the bearer to commit treason. When Macbeth is given this title, the reader can anticipate that he will begin to act against the king. It is the first sign that the witches prophecy is coming true, and sets the whole of Macbeth's murderous behavior in motion.