What happens to the original Thane of Cawdor can be determined at the end of Act 1, scene 2; there we are told that the Thane ofCawdor leads a rebellious army against King Duncan, but he is defeated by the army lead by Macbeth and Banquo. Macbeth doesn't know that the Thane of Cawdor is a traitor until scene three, but nevertheless, the Thane of Cawdor is captured, and when King Duncan is told of this, he states,
No more that Thane of Cawdor shall deceive
Our bosom interest. Go pronounce his present death
And with his former title greet Macbeth.
In other words, King Duncan orders that the Thane of Cawdor be executed and that his title be given to Macbeth for his valor and victory in the war. And in scene 4, we find out that the Thane of Cawdor has been executed.
right as he was a traitor he was killed
Duncan orders Thane of Cawdor's execution.