Nightmare Fuel / Sakigake!! Otokojuku

• The face that Edogawa makes whenever he uses his Face Reversal, even if he is a Paper Tiger, as it is consistently drawn in a graphic and horrifying manner, as it features his now bloodshot eyes open in a hateful and murderous glare, the veins in his face bulging out as if he's literally filled with nothing but pure rage, and his teeth bared like he's going to bite out somebody's throat. It's drawn in an even more horrifying fashion in the anime◊, and he also has glowing red eyes as well.
• The backstory of Date Omito, where he's shown having his now signature face scars of six forgotten virtues carved into his face by an instructor, who he then stabs through the head of by stabbing him in the mouth with a spear, with the spear coming out of the other side of his head being shown. The anime makes this slightly less gruesome by having Date stab him through the torso instead.
• Daigouin Jaki's Deadly Wind Vacuum Wave, due to it's effect of reducing whoever gets hit by it to their bones. It doesn't help that he will sometimes selectively mutilate his opponents with this technique, so he can kill people without them even realizing it.
• Todo Hyoei's grudge against Edajima Heihachi is this when you think about it, as it is shown that Todo will do literally whatever it takes to torment and humiliate Edajima Heihachi, be it by trying to have him killed or tortured, and he also possesses more than enough power and resources to do so. His plot during the Seven Tusks of Darkness arc stands out as particularly sadistic and heinous, given that he kidnaps Edajima and traps him in a satellite in space with a monitor that shows his students battling in the Seven Tusks of Darkness, and he reveals that his intention is to force Edajima to watch his students die while he's unable to do anything about it, knowing how much he cares for his students. Thankfully Edajima manages to escape from the satellite and return to earth. His reason for doing so is also terrifying due to how petty it is, as Edajima simply just attacked him after he ran over his friend Wang. It should also be kept in mind that Edajima Heihachi was literally just 11-years old at the start of their rivalry while Todo Hyoei was already a full grown man.
• Hien CLIII, full stop, due to his Faux Affably Evil nature, as well as his battleground, the Chamber of Intelligent Phosphorent Light. Which he demonstrates the hazards of by stomping on Raiden CLIII, causing him to draw blood, which causes the Man-Eating Hades Fireflies to swarm over him and devour him alive, leaving behind only his bones. It should also be specified that he looks exactly like the previous Hien does, making his actions all the more jarring.