These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Complete Monster: Ryogaku, the first thing he does is inject the heroes with a virus in order to make them surrender, but he really crosses into this by their next fight, when he has a monster experiment try to kill Nataku's father with his mother inside its belly in order to prevent attacks. The kicker, the monster is actually his own son, Bagen, who he made into a mindless monster in the false assurance that he'd be made stronger and shows no concern when he's dying. He gets one of a Karmic Death from his own monster who squashes him like a bug.
Ryukan, a Yandere with a unhealthy fixation to Sengyoku, stalking her in the bathroom and such, but then we learn he sicced a phoenix to chase her around causing her phobia of birds when she refused him which is played straight and tried to do a double suicide when outmatched by Ryukitsu, but it's the psychological trauma that stands out.
Dakki in the anime at least, having no justifications for her disturbing acts other than For the Evulz, understandably she gets a Karmic Death by the end of the series
Nightmare Fuel - Dakki and Kibi making Hakuyūkō into hamburgers (in cooking show format, no less) and then feeding him to his own father. This becomes a plot point when Ki Shou eventually dies of malnourishment after he refuses to eat anything after that incident.
Also a shot of her eating the flesh of corpses of the snake pit victims, looking like she was in ecstasy.
Dakki's torture devices, such as the Houraku.
Although played for comedy, the talking sword can feel pain. Think about it.
Bagen the human paopei's appearance will give you nightmares.
Zhou's enhanced appearance, unlike the manga where it was a rock gorilla, is a demonic looking monster completely mindless
Jyoka has been shaping the world according to her wishes and mercilessly destroying it every time something goes in a slightly different direction. And she has been doing it for millions of years.
Tear Jerker- Despite being a light hearted story with plenty of comedy,the entire second half of Houshin Engi fits this trope.
What an Idiot: Kishou gives his youngest son Raishinshi to a sennin who shows up from nowhere and of whom he knows nothing about. The aforementioned sennin proves to be an abusive Mad Scientist who puts young Raishinshi through hell and practically imprisons him in the floating sennin world, cutting off any means of escape or contact with his family and ultimately feeds him a fruit which changes his biological makeup, causing him to grow fangs and wings. Raishinshi escapes, but is so ashamed of his new body that he feels he cannot return home like that.
So Go, the real Dakki's father, leaves his rather plain daughter with the "palace beauticians" (Kijin and Kibi in disguise). After the fox sennyo possesses So Dakki's body, the girl becomes completely unrecognizable. So Go marvels at the transformation, but ends up actually believing that a woman who looks and acts absolutely nothing like So Dakki (even her voice is different) is in fact his daughter.
Too Cool to Live: Bunchu, Hiko, most of the 12 Elite Sennin, Tsuten Kyoshu, Hakuyuko, Cho Komei Tenko and Ki Sho, basically anyone who overshadows Taikobou as a hero.