In short, the premise of this story is what carries most of it. The switch from a standard-looking MHA-esque hero story to Animong Us is an interesting start, and the series is at its best when it follows this plotline. I won't pretend that the characters as a whole are incredibly developed or the events are perfectly believable. That said, the first half or so of the show keeps interest and momentum by showing the battle of wits between an extremely smart and ruthless killer (which isn't a bad thing by itself, as some other reviews assert: not every protagonist has to be a hero) and an almost extremely smart detective, with just enough funny dialogue and okay backstory to keep the characters from getting unlikeable. It stays just within suspension of disbelief and gives a darkly fun dynamic that isn't seen that much in other anime.
Then along comes the second half, where the writers seemingly lose their nerve and turn a once fairly unique story into a severely generic one. My main complaint is the decision to suddenly make every single person she goes after far worse than she is. In addition to the villains themselves being unnecessary and generally less interesting than past victims, this completely removes any moral ambiguity and turns Nana from a complicated character with a sadistic streak and a badly skewed moral code into a bog standard anti-hero fighting the "villain of the week". In short, what starts out as a heavy subversion of this sort of school superhero/villain story ends up turning back into one played fairly straight. There are other issues, but this is by far the most severe, and everything shown about the possible second season makes it seem like this would only get worse.
In short, watch the first half for the cool kills and investigations, then lower your expectations for the rest.
Then along comes the second half, where the writers seemingly lose their nerve and turn a once fairly unique story into a severely generic one. My main complaint is the decision to suddenly make every single person she goes after far worse than she is. In addition to the villains themselves being unnecessary and generally less interesting than past victims, this completely removes any moral ambiguity and turns Nana from a complicated character with a sadistic streak and a badly skewed moral code into a bog standard anti-hero fighting the "villain of the week". In short, what starts out as a heavy subversion of this sort of school superhero/villain story ends up turning back into one played fairly straight. There are other issues, but this is by far the most severe, and everything shown about the possible second season makes it seem like this would only get worse.
In short, watch the first half for the cool kills and investigations, then lower your expectations for the rest.