you may of noticed at times when it gets really dark the eyes of some people will go pure black, every single time it happens its always with people who have killed or would have killed someone, you can see it with the MC as the story progresses, but is really apparant with the chapter about the little kid being bullied, they do the stabbing shit and now that bullied kids eyes are pure black because he was willing and tried to actually kill the kid. You can see the same thing happen in "OUT!" during the chapter about the street fight when a group get jumped in the middle of the day, one kid has normal eyes but out of desperation and anger to protect his friends he swings a bat with the force to kill and now his eyes are starting to get darker