"One of the main rules of writting. Dont fucking kill off one of your characters without an important reason. Dont think that you're doing something revolutionary by building up a chracter just to have them die and expect the fans to accept it."
...I mean, the work IS entirely about the nihilistic circumstances of life; death comes suddenly and unfairly, and can happen regardless of anyone's "importance" (hence all the flashback panels reminding you of the sheer SIZE of the life being ended-for-no-reason in this chapter), and it wouldn't make that point as impactfully if it did anything less than kill the most important character possible.
It's an art piece about life, not a piece of escapist fantasy. It's supposed to make you reflect on reality, not escape it. The death in this chapter is a logical continuation of the themes of the work (not that it didn't catch me off guard or make me feel sad).
So yes, you said "don't kill off one of your characters without an important reason", and there is an important reason. The story is following through on its central themes. There's nothing more important to the art piece than for it to do that.