He was a 'traitor' who betrayed his country by selling military secrets to the enemy.
He 'turned traitor'. [=he became a traitor; he betrayed his country/friends]
She has been called a 'traitor' to the liberal party's cause.
(..) and protect our people... from the depredation of the Vikings. That is not enough! I must also expose a traitor in our midst. Surely there are no traitors in this hall, sire. Cousin Egbert, why have your lands... never been attacked (..)1958 The Vikings