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Nightmare Fuel: Majin Tantei Nougami Neuro
Sicks is basically an incarnation of this. From the hour he was born, in which he killed all the other babies in his nursery room just for some peace and quiet, he has been pure evil. He has people killed in the most horrific way possible, two notable cases in which he has a failed Mook saw himself to death because he wanted to see him die and a hapless man to vomit all of his blood in the shape of a 6, with the threat of killing his family. The kicker is that the family is Driven to Suicide anyway due to a lie that the father never loved them and that he abandoned them. Sicks' ultimate goal is the annihilation of mankind, not due to superiority or prejudice, but natural selection at work, as he sees his New Bloodline as the next step in evolution while humanity is merely a dead end. He's not bluffing either, he's driven Japan to brink population extinction with his minions terrorist acts. All of which are nasty by themselves.
DR has his victims be drown in vicious flash floods shaped like monsters, considering how close to real life floods in cities it is (except for the extensive knowledge of causing flash floods in the first place) how sudden it can kill people.
The fate of Tierra's victims is absolutely terrifying. He took 1,000 people and buried them up to their necks then gagged and blindfolded them. And then Tierra and Neuro proceed to have a battle complete with falling debris and a crumbling buildings around these people while they are completely unprotected and unaware of what's happening.
Episode 24 and those stone boxes, which crush people to death.
Case in point: their debut revealed that they had spent several years impersonating someone's kindly old grandmother after the original passed away, and nobody could tell the difference - Neuro's demonic abilities only picked up that something was amiss when they were pretending to be a corpse. And the red glass boxes that are X's calling cards are filled with the liquefied remains of their victims.