turn traitor
verb1
To become a traitor, by betraying a person, group, or one's country.
- For a time, it was the only Royalist stronghold between London and Exeter, but it fell at last when a member of the garrison turned traitor and admitted the Parliamentary besiegers who destroyed it with gunpowder.1980, AA Book of British Villages, Drive Publications Ltd, page 134, about Corfe Castle: