double agent 
noun
An agent who pretends to act as a spy for one country or organization while in fact acting on behalf of an enemy: a suspected double agent...
- It was when I challenged him about the double agent who remained his friend despite betraying his country and causing the deaths of many of their fellow spies.
- Is the tailor an agent, a double agent, an innocent or a fantasist?
- Much of what he sold involved US counterintelligence and electronic surveillance techniques, information on active double agents, and documents related to nuclear weapons technology.