Just after Kyo's fight with Nobunaga, when Kyoshiro has taken over Kyo's body, Hishigi shows up to stop Kyo from attacking. He laments that their deaths will not be painless because they have caught him without his sword, and his expression is terrifying. Sasuke panics.
In their final battle, a few of the heroes' comments trivialize Fubuki's suffering. Hishigi does not take it well.
The Devil Eyes in general, especially the fact that they are eating Hishigi alive, and are capable of taking over the minds of their hosts (demonstrated with Akari, and Hishigi's worst fear when he implanted them).
Hishigi's experiments are creepy as shit, but at least he's doing them for The Needs of the Many. Dr. White, whom Sasuke encounters on his own earlier, is a Mad Scientist who creates mutated Artificial Humans and then keeps them imprisoned underground, vivisecting and otherwise torturing them for his own fucked up entertainment and desire to play god.
The rare breed children in Hishigi's lab swarming Sasuke and self-destructing, all talking about how he needs to die with them. This leads to...
Shindara's death. The lower half of his body is completely destroyed in the explosion that he shields Sasuke from, to the point that even his regeneration is useless.
A lot of the earlier fights in the series are gory to a T, especially since the main characters had no moral restraints back then
The Demon Eyes look cool on Kyo, but do not look right on anyone else, invoking one hell of an Uncanny Valley effect - especially with the child experiment "failures", who look extremely off.
The True Demon Eyes are completely red and really live up to the name
The Former Crimson King, a tyrant with a god complex who is much more dangerous than he appears, the fact he forces Synchronization onto priestesses for his plans with heavily implied perverse undertones, not to mention his other crimes. And then there's the intimidation aura he does to Yuya of several hands groping her...
When Kyo is badly injured and collapses in his fight against Kyoshiro, the king appears to invite him into a Lotus-Eater Machine to forget all of his worries... only to try to murder him, made much worse since Kyo is a young child in the subconscious.
When Akira fights Tokito, he uses his Inferno's Chill attack on himself to release pressure points that Julian activated with his needles earlier. This allows him to exceed the limits of his body for about five minutes, even opening his eyes and giving him hyperawareness. The nightmare fuel part is that this destroys him — within one minute, he's curled up on the ground screaming in pain, and he has to get up and keep fighting because Tokito breaks out of his Heaven's Chill technique and inflicts even more damage on him. We get the lovely image of Akira's muscles ripping apart beneath his skin and his bones shattering from the strain. He tells Benitora later that he will most likely never be able to fight again — Yuan's siblings are baffled that he can even stand with his left leg broken as badly as it is.