This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / GoodFellas
Two words: Tommy DeVito.
Yup, Tommy DeVito
Even worse: Scorsese had to tone down some of the things Tommy did in real life...
Tommy's execution is pretty horrifying, especially as he realizes what's about to happen a fraction of a second before a bullet blows off half his face.
The scene where Jimmy starts thinking about whacking Morrie and all the other hired mooks. He just sits there smoking but then the 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. It's early on in the film, and highly disturbing.
After everything, after all the horrible things he's been a part of, all Hill really cares about is the loss of his gangster status. He doesn't care that he's been partially responsible for the murder and ruining the lives of countless innocent people, all he ever gave a fuck about was himself. At least Tommy's sociopathy was out on the surface.
This was intentional, Scorsese got complaints at the previews and he said that the audience should be angry, "The nerve of this guy" and that people should question the mentality and values of the culture that makes being a "schnook" such a bad thing.
One of the things that really drives home the casual brutality of mobster mentality is in the famous "how am I funny?" scene. After Henry realizes Tommy is just messing with him, and everyone laughs it off, a waiter comes up to talk to Tommy about an outstanding bill he has, and Tommy gets pissed, and ends up smashing a bottle over the poor guy's head, and all the other mobsters don't just act like assaulting someone for basically no reason is no big deal, it's funny to them, even though just moments ago they were honestly afraid Tommy was going to possibly murder Henry for no reason other than accidentally offending him. Also, who's to say Tommy wasn't serious, and just changed his mind when Henry called him out?