The Thane of Cawdor is captured and sentenced to death.
At the very beginning of the play, we learn about a big battle. Macbeth fought in this battle, and fought valiantly. This is the same battle where the Thane of Cawdor distinguished himself by committing treason.
The thane of Cawdor, began a dismal conflict;
Till that Bellona's bridegroom, lapp'd in proof,
Confronted him with self-comparisons,
Point against point rebellious, arm 'gainst arm.
Curbing his lavish spirit: and, to conclude,
The victory fell on us. (Act 1, Scene 2)
King Duncan knows what to do about this act of treachery. He is going to reward the valiant Macbeth, and punish the cowardly traitor, the Thane of Cawdor. He is pleased to learn that the battle finally went in their favor. He decides to give Macbeth the title that once belonged to the Thane of Cawdor.
No more that thane of Cawdor shall deceive
Our bosom interest: go pronounce his present death,
And with his former title greet Macbeth. (Act 1, Scene 2)
Either this is some coincidence, irony, or that title is cursed. The former Thane of Cawdor betrayed Duncan and Scotland, and Macbeth is about to do the same thing. Macbeth has been told by three witches that he will become Thane of Cawdor and king. When Duncan does not name him king, Macbeth thinks about taking matters into his own hands. He really wants to be king.
Macbeth never gave Duncan any reason to doubt him before. As far as Duncan knew, he was a valiant and honest soldier, loyal to king and country. Yet the witches have lit a fire under Macbeth. He wants to become king, and if that means he has to kill Duncan to do it, he will. Apparently, Duncan will be betrayed by two Thanes of Cawdor in a row!