In Act 1 Scene 2, King Duncan gets to know about the triumph of Macbeth over the Merciless MacDonwald. The Thane of Cawdor was a traitor so the king order his execution and for Macbeth to become the new Thane of Cawdor.

DUNCAN
No more that thane of Cawdor shall deceive
Our bosom interest: go pronounce his present death,
And with his former title greet Macbeth.