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I really enjoyed Cormoran and Robin. He's got some annoying hangups, but he is obviously lovable. I love him because he hates Christmas (like I do). But he's also a jerk to people who are nice to him. Do better, Cormoran.
I love Robin, but she is a Mary Sue. Perhaps some readers think that good authors don't write those. But they do.

I really did not enjoy the author's making the bad guy monstrous. Again. Rowling, psychopaths exist, but they are rare. The worst atrocities in the world are overwhelmingly committed by ordinary human people. You sort of got it right when you wrote about the Death Eaters. Probably because you based them on a real group - Nazis. They were Death Eaters even during Voldemort's absent years, because they had created social norms that made their deeds normal.
Social norms like these are the reason that soldiers kill. They are the reason American cops kill, and target Black people.
But the bad guys in the Cormoran Stike books kill because they are mentally ill lone wolves, are physically unappealing, and nobody loves them or wants to f*ck them. Monsters. This kind of thing is really harmful to people with mental illness, you know, Rowling.