Duncan sentences the original Thane of Cawdor to be executed. He does this in act 1, scene 2. This scene is a great scene to read and/or listen to. Duncan is asking various people how the battle went, and all of the men give huge, glorious praises to the acts of Macbeth while bad-mouthing any and all of the enemy's leadership. Ross is the character that announces to Duncan that the Thane of Cawdor is a traitor.

Norway himself, with terrible numbers,

Assisted by that most disloyal traitor,

The thane of Cawdor, began a dismal conflict,

No more that thane of Cawdor shall deceive

Our bosom interest: go pronounce his present death,

And with his former title greet Macbeth.

The former Thane of Cawdor is executed for treachery in act 1 of the play Macbeth.

In act 1, scene 2, Ross informs King Duncan of Macbeth and Banquo's heroics by recalling how they defeated the Norwegian forces, which were aided by the treacherous Thane of Cawdor. After learning about the Thane of Cawdor's treachery, King Duncan announces that he will be executed for aiding the king of Norway and gives Macbeth the title Thane of Cawdor.

At the beginning of act 1, scene 4, King Duncan asks Malcolm if the former Thane of Cawdor has been executed. Malcolm responds by informing his father that he spoke with someone who witnessed the former Thane of Cawdor's execution, and they said he expressed remorse for his actions and openly repented for his treachery before dying. King Duncan then mentions that there is no way of reading a man's mind and is upset that he completely trusted the former Thane of Cawdor.

It is the original Thane of Cawdor, who has committed treason by aligning himself with the rebel cause and the Norwegian army which has invaded Scotland. His title is given to Macbeth by King Duncan, which is ironic, as Macbeth himself shortly commits treason by murdering his king.