@Cenholi  Their twisted nature is the main leverage of the story. 
In the end ,they both admitted their faults and grew up. I will applaud for Mathias in the later part of the story, not because he is hot but he became a reliable husband and partner for Layla. Also Layla admitted fully that she was in love with him from the very 1st time. 
So it's not Stockholm syndrome or anything. I get it, it gets very messy in the middle but as reader I don't like these type of one track mind people who don't devle into the actual story or characters psychology. Nothing is B&W.

@Jiji please don't read stories with this one track mind. Its like people are black and white minded overall... Even you are not that, not even me. 
It's  story with dark setting.... Always don't rationalise people's action this way. They both are complex.  Understand the character dynamics. I am saying this without justifying both of them, they both have their fair share of faults and thats make them human. Its just a story... And Mathiass did gave her the choice in the last....If you want bash the story either way, that's a different story. Read rom-com if that gives you peace.

@Cenholi  His upbringing was not normal. He doesn't process feelings normally. He got everything perfect, only exception is layla. That  shocked his core. Even ritte told claudine not to provoke mathias so he doesn't realize he loves layla. Mathiass thinks its his lu** first hand for layla  but later thought his father and grandfather both had mistresses who they really loved, so he thinks its they way of life for an aristocrat like him. His marriage was fixed by his family way before his understanding , he had no say to it. Same goes for Claudine. She also loves Ritte,  mathias's cousin but with mathias for the social acceptation. That engagement is engagement of convenience. 
Layla madly  loves him. But she has her pride. Layla could have told him too, bcz in the later chapters everything was visible before her eyes that mathias loves her. She loves him at the same time she can't have him for they status gap, that's why she throws tantrum always. At the same time, if mathias wouldn't force her, he would never get anything from her. 
Wait for the ending, when mathias will realize he actually loves her what he will do is very satisfying including breaking the engagement. In the mean time, it becomes very toxic, they both are. Its a dark romance story fellas.

@KittyCat  nicely put...
But kayle actually was not her boy friend. They were friends. They know each other for a long time. That sense of comfort can be misunderstood sometimes. And whats better to be with your best friend than a stranger in marriage. I don't like how Kyle pushed her boundary when she clearly told him that she doesn't feel anything romantic between them.... Layla always loved Mathiass. They both are toxic in their own way...

@Jiji She likes him. In fact, she loves him. She doesn't want to admit and also she has ton's of reasons not to. But Her denial phase is longer than ever. It was proved in the novel. She admitted it in the final chapters. 
They both r wrong. You will understand in the upcoming chapters how she enjoys him, at the same time wants to be free from him. She will realize she loves him way before mathias. But right now it is what it is...