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Nightmare Fuel: Good Fellas
Two words: Tommy DeVito. He is generally frightening when he's mad. And the most chilling in Tommy is that does not tolerate words misinterpreted by himself, not to mention his twisted sense of humor, notably in the grave digging scene.
If you pay attention to the personality of Tommy, is pure Fridge Horror. He is incredibly irrational and paranoid, and any word misunderstood by himself can lead to murder. So here, it is very likely already committed other killings or beatings before the movie. Worse, you can imagine how he must have been in school?
Then there is the Billy's brutal death scene, where Tommy hits him until he bled and even some blood falls to his face, most disturbing of all is that he wanted to continue the beating, even when his partners stopped him. Even Henry is visibly disturbed for this action.
The scene where we see what happens to these gangsters when they go to prison: they live like kings. The prisons have been reformed quite a bit since then, but even a prison system one tenth as corruptable as then one presented in the film lets career-complete-monsters off sickeningly lightly.
The scene where Jimmy starts thinking about whacking Morrie and all the other hired mooks. He just sits there smoking but then music goes up and you know he's up to no good.
DeNiro's eyes in that scene. They just say, with such incredible subtlety, "Time to go on a killing spree."
The way Henry's father beats his son when he's a teenager is brutally realistic setting for early in the film, and highly disturbing.