Chapters 1 to 4 are based on this:
https://en.wikipedia.org/wiki/Murder_of_Junko_Furuta
The most disgusting thing about the case is that the perpetrators received incredibly light sentences and are all currently free men. Some of them even brag about it, and continue to commit crime and assault people. The mother of one of the boys defaced Junko's grave because she "ruined her son's life.", while he himself spends money lavishly so he doesn't have to give any of it to Junko's family.
Honestly, the aftermath of the crime is infinitely more fucked up than anything Uziga Waita has penned.