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.
YMMV: Heart Core
Asshole Victim: It's hard to feel sorry for the first humans that Ame killed in New Ayers while in disguise, considering that they were not only going to mug her, but also rape her. And since it was mentioned that they were killed in an area where these things are not too uncommon, one can consider this proof that this wasn't their first time doing something similar to other people.
Complete Monster: Royce is, as one fan so eloquently put it, a "douche-fag". His crimes include causing a massive massacre against the beastman population, plotting to murder his own wife in order to become stronger, stealing power from his daughter through painful rituals that causes permanent and large scars, hiring a complete maniac (even by demon standards) to be his second in command, and trying to crush Asmodai under a massive chandelier for the sake of cheap laughs.
Irony: Carval's sin is supposed to be Sloth, yet he acts like a hyperactive child with a serious case of pyromania.
Word of God knows of this, but also subverts this, since "sloth" can either mean laziness or apathy. While the former would, in no way, describe Carval, the latter goes hand-in-hand with his love of explosive ordinance.
Nightmare Fuel: Imagine that you are helplessly watching your sister be brutally injured and delimbed by her demonic father. Then imagine him Mind Rape her with the help of satanic magic in almost an instant. Then imagine that very sister run towards you, sporting an insane grin on her face before shoving her hand through the back of your head. Sweet dreams.
Tear Jerker: Remember when Ame was mindcontrolled by Royce so that he could make her brutalize her own sister Suko? That in itself was pretty bad, but what makes the situation even worse is that Suko has not forgiven Ame for it. Only time can tell if the remade chapters will make this situation better or worse.
Another from History Of Nifelheim, the prequel of Heart Core. After being mortally wounded by Royce, Lilium begs to Asmodai to take care of her daughter, wanting her to be safe from her father. She dies shortly after as her Heartcore turns into the child that would later become Ame. To make matters worse, Asmodai had an unrequited love for Lilium, having never admitted his true feelings for her, even in her dying moment.