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 they have caught him without his sword, and then makes a terrifying face.
In their final battle, the heroes trivialize Fubuki's suffering. Hishigi does not take that well.
Shindara's death. The lower half of his body is completely destroyed to the point that even his regeneration is useless.
The rare breed children in Hishigi's lab all swarming Sasuke and self-destructing, all talking about how he needs to die with them.
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, while they look cool on Kyo, 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 Version is completely red and demonic looking
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 and then there's the intimidation aura he does to Yuya of several hands groping her
when Kyo is defeated by Kyoshiro, he 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 he's a child in the subconscious.