Throughout chapters 12 and 13, the reader gathers the impression that Isabella's initial elopement and consequential marriage to Heathcliff is rather falsified and lacking in any love or passion.
As is the case with the majority of the novel, these chapters are narrated to us by Nelly, who makes for an excellent narrator due to her ordinariness. Catherine is a very awkward and irritating character, but Nelly is so resolutely unsympathetic with her that our exasperation with her is deflected. She also forces the reader to evaluate every event for himself, as her judgement cannot be trusted. Through observations and a verbatim epistle from Isabella, Nelly reveals to the reader how Heathcliff manipulates Isabella, and finds a way into her affections. .
In chapter 10, Isabella first tells Catherine of her love for Heathcliff, which is "more than ever [Catherine] loved Edgar." And despite both Catherine and Nelly doing their best to dissuade her, Isabella stubbornly refuses to accept that Heathcliff really is a "fierce, pitiless, wolfish man.".
Catherine then informs Heathcliff of Isabella's feelings, and his response is rather brutal. He refers to her as a "creature," showing how insignificant she is to him. He then says that he would "wrench [her nails] off her fingers if they ever menaced [him]." And then as if the reader needed further convincing of his abhorrence of Isabella, Heathcliff comments that he likes her "too ill to attempt" living with her, in a "very ghoulish fashion". And so it is surprising that Heathcliff defies Catherine's order, that he "must" leave Isabella alone.
The true motivation behind Heathcliff's behaviour is suggested to the reader before Heathcliff actually admits it himself. In chapter 10, he clarifies with Catherine that Isabella is Edgar's heir, and Nelly suspected that he often "recalled" this fact during "the course of the evening.