what does king duncan say about the traitor cawdor?why is duncan's comment part of motif?

He has said earlier in the play that the Thane of Cawdor will be executed for his rebellious actions. Now he finds out that the Thane has been executed and is thankful for that being done. He has dealt with a traitor. We know that this is a violent country, this being the most violent of Shakespeare's plays. This is one of the motifs - violence