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Chloe: Lex is dead.
Oliver: What does Clark think?
Chloe: That Winslow turned on Lex. Was against Lex and killed him?
Comments  
Hi Madhulk. Yes. When people turn on you, they attack you or speak angrily to you.
Thanks, Anton.
Students: Are you brave enough to let our tutors analyse your pronunciation?
You're welcome. Emotion: wink
No, it doesn't mean to get angry at someone.

It means to betray someone.
Hi, Mschufman. I can't find the meaning you mentioned.

http://dictionary.reference.com/browse/turn%20on

CollinsCobuild tells me the following: If you say that someone turns on a particular way of behaving, you mean that they suddenly start behaving in that way, and you are often also suggesting that this is insincere. [INFORMAL]

But in that context, I guess explanation given is appropriate. Nonetheless, without more context we can't say it unambigously.

F.
Students: We have free audio pronunciation exercises.
Turn on.

there are many forms with distinct meanings but in this context it means something similar to this definition from dictionary.com

i. Also, turn upon. to become suddenly hostile to: The dog turned on its owner. Not exactly betrayal, although this could result.