What is the scariest NON-horror movie you have ever seen?

One of those realistic "hardcore thrillers" I guess I like to call them.

Stuff like Death Sentence, I Saw the Devil, Training Day

Threads

That movie more just weirded me out. The part with the those guys on roller skates is creepy though.

Natural Born Killers
Batman Returns (As a kid it freaked me out)

I remember having literal nightmares about Selena’s resurrection scene.

For me personally the most unsettling scene in Natural Born Killers is when they deconstruct the American family sitcom with Juliette Lewis' family. Oliver Stone could have played it straight, but purposefully cast generally comedic actors like comedian Rodney Dangerfield and Edie McClurg as the parents and had them deliver their dialog like they were in a comedy and not a drama. In Dangerfield's case, he was not given a complete script, and took on the task of writing the filthiest things his characters says after Stone informed him that he was playing "the father from Hell." And indeed, he is. While still a disturbing scene, it makes for an interesting experiment and commentary on people's attitudes regarding comedy, in this instance iconic funnyman Rodney Dangerfield is playing a horrible person that should be behind bars serving a very long sentence, and Stone is asking if the viewer can still find the person amusing due to the actor's performance despite the character being complete and utter scum. It does leave an impression. 

Had this scene been in the film, I would have placed it over the one above, which is where Harrelson and Lewis are on trial for the death of a young man, and Ashley Judd plays the girlfriend of the deceased and the witness. Through most of the scene, Harrelson's character is rather calm and smug about his actions, but when Judd berates him, he loses it and just ends her in the room, which sparks uncontrollable laughter from Lewis' Mallory Knox. 

Contagion

Requiem for a Dream