It's a common trope of many different genres of fiction that one character becomes a "surprise traitor": someone initially presented as an ally/friend of the protagonists, but is revealed later to be a two-faced antagonist. In wuxia, whom do you consider the most shocking turncoat...where you really didn't see it coming and were truly surprised?
My nominee: Beggar's Union Lawgiver Elder Bak Sai Geng. This guy was supposed to be the honorable enforcer of the laws in the Beggar's Union, and seemed to be very loyal to Kiu Fung even in the midst of the controversy surrounding Kiu Fung's Khitan heritage. When it was revealed that he had always been secretly in league with Madame Ma to undermine Kiu Fung, I was truly shocked.