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: Violinist of Hameln
Complete Monster: While most of the mazoku tend to be at least somewhat Laughably Evil, and remain entertaining despite being thoroughly monstrous, Great Demon King Chestra definitely crosses the line to this. It doesn't really hit you what a Complete Monster he really is until the flashback to Pandora: after he got out his body, but not his magical power, from the Pandora's Box, he pretended to be her apparent loving husband for a couple of years, so he can trick her into opening said box. Bad enough? Wait, there is more. His children with Pandora? Explicitly meant to serve as food to restore his power. Nearly all the shit they were put through? Orchestrated by him to make them magic-rich and tasty. And then when the bastard let all the Mazoku out on Pandora's front lawn, he stops the mooks from tearing her apart...because he wants to drink the blood of a holy maiden. Chestra is a complete DICK.
Evil Is Sexy: The manga clearly subscribes to this trope - Sizer's normal outfit is very modest, while "Dark Sizer" is Stripperific.
Ho Yay: Done very intentionally in the manga with Hamel and Raiel - the other characters flat out accuse them of being gay a few times. And let's not even get into Olin harassing Hamel. And then there's that scene when Clarinet is introduced, and the heavily implied crush Clari may have had towards either Lute or Horn (or both? It's never truly clarified) for most of his life.
Watanabe hangs so many damned lampshades in the manga that it's a wonder no one's mistaken it for some kind of deranged chandelier. He draws roses in the background during the yaoi parodies, for crying out loud.
Examples of Ho Yay above at no point go beyond gags, though. In Schelkunchik, on the other hand, you cannot help but suspect this, when Schel and Great fall into the same roles that Flute and Hamel played in the original series.
Iron Woobie: The entire Team Hamel and some of the supporting cast. After all the horror and torment piled on them, it's amazing that they aren't suicidal wrecks with the temporary exception of Sizer, anyway.
Magnificent Bastard: Arguably Chestra. Definitely Vocal. The sheer evil he puts Orgel through...
Narm: A lot of people consider the anime just as hilarious as the manga, only in a different way.
Anything involving Lute (in the manga). He's tortured to death protecting his mother, Flute and Sforzando— having his eyes cut, his throat and legs stabbed, and his arms crushed. He is then possessed by Hell King Bass, and abused as his puppet for fifteen years... dying again upon his release.
Sizer's backstory. Kidnapped by and raised among the Mazoku since she was a baby, with Ocarina as her only friend. Forced to destroy countries since the age of six. Being lied to that her mother abandoned her in favor of her twin brother, then finally meeting her mother... who is sealed inside crystal. Spending the next 10+ years of her life massacring humans and rising through the Mazoku ranks, pretending to be on their side while waiting for her chance to strike back.