7. Frank Fontaine (Bioshock)
Throughout the game, you only had Atlas as your partner to guide you and help you understand the chaos that has befallen Rapture. You even help him try to save his wife and child. In the end, you had to kill him because there was no family and Atlas did not exist. You were talking to Frank Fontaine the entire time and he manipulated you to do his bidding through using the "would you kindly" trigger phrase. In fact, your whole life was a lie.
Sometimes we don't have problems killing traitors because they are usually stock characters. However, Fontaine is an excellent example of a character players were floored upon realizing his treachery.
