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.