4. The Departed
The main reason this "happy" ending (at least in light of all that bloodshed) came out of nowhere is because the original Hong Kong film on which it is based, Infernal Affairs, has the corrupt cop killing the good guy and essentially getting away with it. Fans of the original were in for something of a surprise in the superb, arguably superior Martin Scorsese remake, which follows the same trajectory except for an additional scene that is tacked on at the end. After undercover cop Billy Costigan (Leonardo DiCaprio) is murdered and undercover criminal Colin Sullivan (Matt Damon) has gone about cleaning up all of his mess, it seems like he has, like in the original, gotten away with it. However, when Sullivan returns to his apartment with some groceries, Sgt. Dignam (Mark Wahlberg), a character who didn't even exist in the original film, is waiting there for him, wearing some print-free shoes and having his gun already aimed. Sullivan gets shot through the head, Dignam flees the scene, and the film ends. Even though it's clearly thrown in to appease American audiences, it still really works, because everything leading up to it allows it to make sense, and Damon played the douchebag card so effectively throughout.