Indefinite article: Turn traitor vs become a traitor

1 The man turned traitor ([dative noun or] adjectival as complement) after he was arrested.

Compare "The man turned traitorous ( adjectival as complement) after he was arrested.

OED

Traitor 3. attributive [noun] or as adj. That is a traitor, traitorous.

1837 A. Tennent Vis. Glencoe 18 Some traitor spy, Meant to betray thee with a lie.

OED:

Turn 16.a. intr. To change one's course, so as to go in a different direction; to deviate.

For example: "Meanderings in the West" - Page 62 Elaine Seavey - 2006

At the town of Wisdom, our road turned west, climbing over St. Joseph Pass

turned west = turned to the west. (NB, here "west" is adverbial)