50 Movies Where Evil Won

Plot: Scientist Matthew Bennell (Donald Sutherland) discovers that the people of San Francisco are being replaced by emotionless clones, ones created by a sinister alien race.

1978's Invasion of the Body Snatchers is one of the all-time great horror remakes. Even if it isn't quite as good as the original film, it does exactly what a remake should do: it expands upon the original movie and finds new ways to explore the concept. It also takes advantage of the more permissive climate of 1970s New Hollywood to deliver a darker, more gruesome take on the story than the original ever could, including that famously horrifying ending. Just thinking of it gives one the shivers. 

By the film's end, much of San Francisco has been replaced, and one of the last uninfected humans, Nancy (Veronica Cartwright), approaches Matthew, who's presumably still uninfected... he isn't. He points at her and lets out the terrible high-pitched scream all of the duplicates use, confirming in the most haunting way imaginable that he's now been replaced with a duplicate.

In the original, they manage to stop the invasion of the body snatchers, but here? The aliens have started sending out pods around the country, strongly suggesting that soon the whole world could be overrun. Endings don't get much bleaker than this.