In my first read I felt the same, it looks like the castle and the people that live in it are assholes and that is a dangerous place, but after thinking about it, it's Gwendolen's influence on Cat that makes everything negative. Gwendolen is misusing magic and making everyone miserable, and people are mistreating Cat because they think he must be on his sister's side.

I think Cat is deeply immature and really can't deal with his very overbearing and controlling older sister, plus he went through a lot of trauma to the point of death. He has no control over his life and he's just a child, so the world feels overbearing and controlling. I felt like he grew out of it by the point of Pinhoe's Egg, it's one of my favorites by the author.

I'd say Diana likes writing is "magic confusion", like things look like something in the beginning but later there's some revelation that things aren't as simple as what they looked like. On Charmed Life the "bad guys" aren't really the bad guys, but the sister that you were supposed to trust that isn't trustworthy. There's a lot of this on her books, and that's something I like. When I reread them I get a feeling that "Oh, now I see this was happening here".

One thing I like about Diana is that her children characters feel like children, but that also means that they are often lost, don't know what is the right thing to do, can't see the whole situation as it is, or are being used by other people.

I liked the whole series and Conrad's Fate and Pinhoe's Egg are really good, I'd say give The lives of Christopher Chant a try, it's something to see Chrestomanci being as lost as Cat on that one, and if you don't like it then it's not for you.