11. Buttons - The Greatest Show on Earth (1952)
Much as I did when using The Clown Murders at #12, I feel like I'm cheating with this one as well. James Stewart (It's A Wonderful Life, Rear Window) plays a clown named Buttons that works for the Ringling Bros. and Barnum & Bailey Circus. He refuses to ever take off his makeup, leading those around him to imagine all sorts of sinister reasons for Buttons' shyness about his face. The reason I feel like I'm cheating? Buttons isn't evil and doesn't kill anyone, but a lot of people think he might be evil. Close enough for me.
How about if I make up for cheating by suggesting you watch Closed for the Season? I enjoyed this film. It's not the best and it's not endlessly filled with evil clowns, but when it does have clowns, they are scary. Wackles, Hackles, and Cackles are the sad, happy, and laughing clowns that stalk our heroine Kristy for a while. Oh, and the carny is a true freak. Watch this film, don't expect too much, and enjoy the clowns.