The Idea Factor - Revenge of the Betrayed
The book starts out with five friends. Soon, they are on the run, with a conspiracy chasing after them thinking they know too much. Early on, one of the friends shoots another at the end of a chapter, seeming to have killed them. The next chapter starts with only that character, the others having left her for dead. But she is in the hands of the conspiracy, with them having just brought her back to life. She then works with the conspiracy against her friends. In the end, the person who shot her did it because they thought she was already working with the conspiracy, but she wasn't. And it's only her work with the conspiracy that brought the others down.