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Nightmare Fuel: The Godfather
Two words: Luca Brasi. He's possibly the most dangerous, cold blooded and sadistic mobster a mafia film has ever had (along with Tommy DeVito of course), besides being arguably the most terrifying character of the franchise. Furthermore, of all the villains in the franchise, Luca seems to be the most unnecessarily evil character, and extremely cruel.
Not to mention some of his many atrocities: killed his own father, 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. What makes him so horrible is that, all of it was done willingly. Luca is little more than a monster, who's only on the "heroes" side because of his devotion to Don Corleone.
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.
In The Part II, the simple fact that Don Ciccio has killed the entire family of Vito and even looking for him when he was a child is just horrible.
Mosca from The Part III is frightening as well. He was deranged and terrifying overall. His methods of murder were pretty brutal.
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 genial 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.