I entered the above answer inadvertently. The Thane of Cawdor is the traitor referred to in Act 1, Scene 2.
Norway himself, with terrible numbers,
Assisted by that most disloyal traitor,
The Thane of Cawdor, began a dismal conflict...
Duncan pronounces the sentence that will leave a vacancy for Macbeth, who has not yet encountered the Three Witches.
No more that Thane of Cawdor shall deceive
Our bosom interest. Go, pronounce his present death,
And with his former title greet Macbeth.
Then in Act 1, Scene 3 Macbeth is greeted as Thane of Cawdor by one of the witches, and later in that scene he is greeted by Ross as the Thane of Cawdor.
Malcolm is Duncan's eldest son. He became stigmatized as a traitor and a patricide after the murder of his father by Macbeth. But he was completely loyal and innocent. Macbeth found it convenient to accuse him of the murder, and he had fled for his life and was unable to defend himself. Before Duncan is assassinated, he proclaims Malcolm Prince of Cumberland, which makes him heir apparent to the Scottish throne.
At the beginning of Act 1, Scene 4 there is further reference to the condemned Thane of Cawdor. As he was being executed:
...very frankly he confessed his treasons,
Implored your Highness' pardon, and set forth
A deep repentance.