This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Godfather
The Godfather (the novel) and general notes about the entire saga
Some of Luca Brasi's actions are really Nightmare Fuel in the book. He dismembered two rival gangsters with an ax slowly and even had his own child, an infant he had with a prostitute, thrown into an incinerator.
The very concept of Corleone and his family, that this close knit and seemingly loving family are actually brutal, vicious murderers and criminals, no matter how clever and generous they may seem on the surface. For all the talk of honor, there's a reason Michael finds himself dying alone and abandoned in Part III.
The idea that by the end of Part II Michael is basically on his own. The once tight family is totally gone. Sonny was ambushed, his father died of old age and his mother too. He had his own brother murdered. Finally his wife has left him. You can imagine how awful this feeling must be, but all we see him do in the final shot is stare and not say a word. And in Part III when things seem to be getting a bit better for him: he has reconciled with Kay and his sister Connie, his reputation is better than before and he is about to leave the mafia business... the Cycle of Revenge gets back at him again. Near the end he dies alone, abandoned by everyone, having reached nothing with his life.
Carlo getting whacked as retribution for the above scene. After being told he's getting put on a plane to Vegas, he's put in a car with Pete Clemenza sitting behind him. Clemenza garrotes him so viciously that he's pulled partly into the backseat and kicks at the windshield, shattering it, which is a good thing because it saves us from witnessing the garrote cutting through the bastard's neck. It doesn't save us from hearing it.
The scene in which Woltz, after actually refusing an offer he couldn't refuse, wakes up in bed the next morning with the bloody head of his favorite horse. Creepier still is that they used the head of a real dead horse.
The mob hits against the remaining rival families while Michael was at church, showing how numb Michael has become at the thought of murder. Tattaglia's death was especially gruesome as he's riddled with bullets while in bed with a prostitute (who also doesn't survive).
Pretty much the entirety of the hospital scene. The sense of urgency and wonder if Don Vito will survive another assassination attempt on his life again as he lies helplessly in his bed. Michael being Genre Savvy tells an oncoming nurse to help relocate him in another room and manages to stand his ground to the oncoming assailants (with the help of Enzo the baker who happens to be stopping by to pay his respects to Don Corleone) at the front of the building by warding them off with a ballsy bluff. The atmosphere for some reason screams Paranoia Fuel.
In The Part II, Don Ciccio killing the entire Vito's family when he was a child.
Likewise, the scene where Vito returned to his hometown to find Don Ciccio, who was old as well as somewhat blind and deaf. This didn't stop Vito from slicing Ciccio's stomach open with a blade to avenge his family, showing just how strong his desire for revenge was.
Though it was well-deserved, the scene in part II where Fredo is killed, while Michael just stands and watches from the lake house. The editing leading up to the scene of all the mobsters dropping like flies, similar to the montage near the end of the first film is part of what makes it so frightening. In addition, the blu-ray menu consists entirely of Michael standing and watching from his lake house.
Mosca from The Part III is frightening as well. He was deranged and terrifying overall. His methods of murder were pretty brutal. Part III's blu-ray menu shows the Vatican banker hanging from a bridge, and nothing more.
The frightening wounds on the victims of the mob war (shown in headlines and newspaper photographs) hit home how bloody and terrifying the "war" is.
What's even worse is that many of them are photos of actual mob hits and murders.
When Michael has a stroke after Zasa's assassination attempt. It all looks so real you'd think Al was having one for real and was rolling with it. It then leads to Tear Jerker territory when before he collapses he shouts Fredo's name.