Aria, the main protagonist, is actively emotionally manipulating everyone around her and playing with the heart of an innocent and well-meaning boy purely for her own enjoyment. And every time we're shown signs of genuine emotion, of sadness, rage, and complete and utter terror, she simply doubles down on her scheming and it's horrifying to watch.
Mielle, her sister, is at first shown to be evil, then displayed as a victim, but then actually makes the first physical move against Aria, and the constant suffering she goes through is always suddenly torn apart by small, innocuous mutterings that suddenly whiplash you with how completely ugly she actually is inside.
And that's just the two main focuses of the story. The father blatantly lies about which daughter did what, the mother is lazy and set the whole story in motion by seducing the father for his money, Oscar's sister is physically and emotionally abusing him, and every single supporting character other than Best Boy and Best Maid are superficial, greedy, or self-absorbed.
So many characters are just completely horrible people, yet the way the author is making them all bounce off of each other is less like watching a train wreck and more like watching a horrific and bloody fight between gladiators in a coliseum, and you just have to keep reading because you just can't stop wondering what these monsters are going to do next.
In fact I consistently find myself actively repulsed by these character's actions sometimes, watching as they pull innocent people into the crossfire, using them as weapons and shields, with Aria being the worst...
And I love it.