I can tell you why people hate this, it's because the genre shift in the first chapter was incredibly done and very impactful. Making you hyped for some Death Note mind games, but then you're immediately served with trash.
I think we should call this the "blue ball effect". Higher expectations will lead to greater disappointments and then greater frustration.
Of course, there's worse manga out there, but at least those are mostly mediocre from beginning to end.