From Longman Dictionary of Contemporary Englishturn traitorturn traitorBETRAYto be disloyal to a person, group, or idea that you have strongly supported before → turn
Examples from the Corpus
turn traitor• He did not appear to be the type which could be persuaded to turn traitor.• Of millions of men and women, only a handful had turned traitor.• The player must then decide whether to turn traitor and eliminate the partner, he added.• Perhaps, by her action, to convince him that she had not turned traitor to the King. 