This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Django Unchained
The Mandingo fights. Also the slave who was torn apart by dogs.
The cold demeanor of the "winning" Mandingo being congratulated by Calvin Candie. He is either a remorseless killer, or, far more likely, he's been doing this for so long he's become totally desensitized to the horrific situation he is in in life.
Also, Candie treats him like he's just done some hard work around the house, and not break a man's arm, gouge out his eyes, and then kill him with a fucking hammer to the face.
Candie telling Big Fred to finish his opponent. With a hammer.
Just about any of the scenes showing the brutality of slavery. Tarantino pulls no punches.
Some people just won't die. And they keep getting shot.
Calvin Candie is really, really creepy and deranged when he is pissed.
The Jump Scare that he causes when he swings the hammer just a few inches from Brunhildes head narrowly missing her and breaking a table in the process is very well done.
The scene where Billy is willing to castrate Django while he was tied up upside down for hours.
Stephen is blood-freezingly calm as he tells Django about how he will send him to the LeQuint Dickey Mining Company, how any atrocity they may inflict to him pales in comparison to that, how for Candie's death he is gonna suffer to the last breath.
"And as a slave of the LeQuint Dickey Mining Company, henceforth until the day you die, all day, every day, you will be swingin' a sledgehammer, turnin' big rocks into little rocks. Now, when you get there, they gonna take away your name, give you a number and a sledgehammer, and say, "Get to work!" One word of sass, they cuts out your tongue. And they good at it, too. You won't bleed out. Oh, they does that real good. They gonna work ya all day, every day 'till your back give out. Then, they gonna hit you in the head with a hammer and throw your ass down the nigger hole. And that will be the story of you, Django."
Also how bleak the situation looked until he managed to escape. One would be forgiven to believe that there is no escape this time.
The first shootout in Candieland, after Candie's death. Even for an awesome moment, it is quite disturbing the fact that is possibly one of the most bloody and brutal shootouts in film history, on par with the Grand Theft Auto V stuff.