"Lolis Getting Killed Left and Right: The Light Novel": The very nature of this series involves a lot of cursed children (a race of lolis) getting indiscriminately killed off.
"NTR Bullet": Volumes 5 and 6. It's infamous for having the main antagonist forcefully kissing Kisara (Rentaro's love interest) and forcing a marriage with her. The said antagonist will later frame Rentaro for murder, forcefully ruin Enju's life by having IISO take her away and forcefully pair her with a stranger named "buddy killer" (implied that she suffered a lot of physical and emotional abuse from "buddy killer"), and Tina Sprout sentenced to death row. Apparently, this caused many of the Chinese light novel translators to drop their translation plans for this series due to how dark and depressing the later novels become. Not to mention, the plot device seen to be really trollish (see Trolling Creator below). Fortunately, volume 6 ended with the villain's plot foiled and Rentaro name cleared and reunited with Kisara, Enju, and Tina.
Trolling Creator: If you are curious why every loli and Rentaro are given really sadistic treatment throughout the light novel, this is why. According to one light novel translator's note, the author happens to be a huge fan of Nitro+ games and Gen Urobuchi's works and Black Bullet is somewhat inspired by Gen's works. The author even originally wanted to have Rentaro having a personality similar to Kiritsugu before ditching that idea since the personality would not fit into the setting he was writing in. The author even throws one of the most controversial plot devices in volumes 5 and 6 where Rentaro's love interest, Kisara, was forced to marry the antagonist and the said antagonist framed Rentaro for murder. Fortunately, that plot device ended being subverted.
The fifth volume author notes confirms that Shiden Kanzaki intends to be the worst writer in light novel history by deliberately trolling his readers. Although he acknowledges his own flaws in his writing:
Shiden Kanzaki: Taking myself as an example, I first go "I want to write this kind of scene!" then I think about "How exactly should the story go to develop into that direction?" through a process of reverse thinking (also known as the induction method) to conceptualize. Conversely, reversing the steps results in what's called the deduction method, but why do I find it impossible to write using the deduction method? If I try to write a story using it, I inevitably end up getting stuck somewhere. Although this is my flaw as a writer, it's not a totally bad thing. The inductive method is very logical so as long as things are arranged sufficiently cleanly, the story will have unity in logic.