I think this
'However 'He turned a traitor" means somebody influenced a traitor to turn, referring to two people, the 'He' as well as the traitor'
introduces a quite rare sense of 'turn'. I agree with the point, but I doubt whether it's likely to contribute to the OP's understanding (except in so far as it might lead Kotfor to regard that usage as more usual than it is!). Besides, the collocation is odd; a person who is turned (in that sense)
becomes a traitor; they're not a traitor until someone turns them. If you 'turned a traitor' you would be taking a traior and making them a loyal citizen again! ;-) The expression
'turn a traitor' just doesn't occur.
(I imagine Google will say it does, if only because of this discussion, but BNC has no cases, and the single one in COCA comes from Shakespeare - who was writing at a time when the language was more fluid; maybe he just added the 'a' for the metre. [Although it's not written metrically, there may be a joke - which I don't understand - involving a 'rude mechanical' breaking into iambic pentameters as a sign of their pretentiousness.])
b