Nightmare Fuel / Inuyashiki

• Hiro Shishigami. A seemingly-normal highschooler that becomes a Serial Killer after being turned into a cyborg. Aside from his family being oblivious to his actions, there's the fact that he has no qualms about killing every member of the families he murders. The first family he kills includes a father bathing his son in the tub. When Hiro shoots him, the man falls on top of his son and Hiro waits until the boy finishes drowning before he leaves. Watching the boy's arms flail in panic and seeing his fingernails tear strips out of his fathers back before finally going still should make it abundantly clear what sort of person Hiro is.
• The whole arc between Chapters 19-27 is about a big and scary Yakuza mobster that drugs and kidnap a woman with very implied intentions to rape her just because he can... and it happens twice! In the first time, she escapes, but is tracked down by the criminals and kidnapped again, with her husband almost being killed during the attack.
  • While wholly deserved, the entire Yakuza organization's fate is quite scary to contemplate: eyes blown out and spinal cords severed, leaving them unable to see, move, feel, or even end their own lives, forced to live out their lives in a state of complete helplessness.
• Shishigami's wholesale slaughter of 2ch posters who trolled his mother into committing suicide. This is frightening on two levels: the first, of course, is Shishigami's lethality, while the other is the sheer number of people involved in the trolling and who they were. They run the gamut from your typical slovenly Otaku to students, businessmen, and others. Shishigami may be unique in having cybernetic superweapons, but he's not the only monster in the guise of a human.
• Chapter 30.5 revolves around Shishigami; Nightmare Fuel is to be expected. What isn't expected is him killing a group of home invaders that already attacked a home he had staked out.
• Chapter 38, fittingly named "Kill You All". Shishigami snaps after his mother's suicide and graphically massacres the news team interviewing his father.
• Shishigami hijacks more than thirty planes and makes most of them crash around Tokyo, killing and injuring tens of thousands.
• Shishigami's planned genocide of Japan's population evokes imagery of mass shootings which can be hard to watch. What's worse is that as long as he sees someone on camera, in person or on camera, he can kill them anywhere at anytime, even if they aren't watching TV or checking their phones.
• Shishigami encouraging a young boy to put his phone to his ear so he can kill him. The relief one feels when his plan fails and the phone hits the ground is almost palpable.