Is that subtelty? Maturity? Leaving it as gray as possible? Sorry, but the way it is handeld now, it's simple "edgy".

Yeah, I didn't like that either, I don't think it was necessary, it was was over the top, and went down further than it needed to be.

As of now, we perveive her as an one-dimensionally evil character with not depth whatsoever.

But again, that's how he sees her, and I don't think she'll get that much more depth either later on (I also haven't seen the latest episode or read the manga), but that's fine and I don't necessarily think is an "excuse" for her to be one dimensional. Who are currently most important people Satoru at the moment? From what I see? Kayo, his Mom, and the killer(s), and while he's trying to observe everyone else to see if they give off indications of being the killer(s), he's also protecting Kayo and his mom and as he sees someone mistreating Kayo, he's simply going to see them evil. If you walk down the street and see someone abusing their spouse, are you going to try figure out their back story and why the abuser's hurting them, or are you going to step in and stop it? For that moment, many people couldn't care less what the reasons for abuse are, they're going to step in. Back story is for later on for the courts and judging the abuser's character, but even then those that witnessed the abuse are not likely to suddenly change their opinion. The story is definitely first person as we only see and hear the thoughts of only Satoru 95% of the time, I want to say this could be first person omniscient due to scenes that could be implied and shown safely that he could assume to know like the head dunking, but then that doesn't explain the teacher and the one guy standing in the classroom (I swear there's a literary term for this that's going over my head).

So damn obviously one-sided conflicts (evil vs. good) are not ok in a proclaimed seinen that should be taken seriously.

How serious are is he supposed to take it? As I mentioned, he sees the abuse, and are most people going to question or change his mind on her because she's given backstory? Short answer, no, he does see this as good vs. evil because it's his perspective.

I doubt that the show/author/director will make any efforts to solve this problem.

Yeah, as I mentioned, I doubt it also, but that's perfectly fine.

Overall, I think this is an issue that is false nitpicking, trying to nitpick when it really isn't an issue. Especially when she's pretty clearly not the main villain (unless she does turn out to be the killer, then I'm obviously wrong), she's not the big of a character to Saturo, and forgetting that this is first person, you can wish all you want to know all about the characters, but Saturo is giving us the info, and he can't observe everything magically.