I was reading a English grammar book and it was mentioned that "turn" is a inchoative verb and if "turn" is used as a inchoative verb and used with a noun then you cannot have an indefinite article with it.
A correct example shown in the book was - I believe you will never turn traitor.
My question is this sentence incorrect (I used "into" preposition in-between) - I believe you will never turn into traitor.
Also, please provide explanation for either answer you may give.
Turn, when inchoative, doesn't need into. The inchoative use is things like turn red or turn evil, meaning to develop a characteristic. This usually takes an adjective as object, but there are several uses where it takes a noun that describes a characteristic of a person, generally representing some sort of diametric change - moving from one thing to its opposite or complement. For example, a common metaphor for a criminal who has turned to working against crime would be:
He was a poacher turned gamekeeper.
A gamekeeper being one whose job it is, among other things, to stop poachers.
Turn traitor is one of these examples, appearing in Oxford as a set phrase. It is, however, the same structure as "turned gamekeeper" in the saying above. It means to develop the characteristics of "being a traitor".
Now, you can use to turn with an argument using into. A witch might turn someone into a toad, after all, or a good man turn into a monster (metaphorically, usually). However, in that case, you do need appropriate articles. Thus, if you want the turn into version, you want:
I believe you will never turn into a traitor.
That, however, suggests the person fundamentally changing, rather than simply betraying someone. They were not a traitor, and then they were, which might seem like the same meaning as turn traitor, but the nuance is definitely different. If someone turns X, it is a decision they have taken, a change in the course of their life perhaps, but if someone turns into X they have generally changed in some fundamental, deep way.
