Heathcliff tells Nelly that Isabella had the delusion that he was a chivalrous man, and he can hardly consider her rational for doing that. When she thought he was in love with her, nothing could dissuade her from that thought, but she found out that he did not love her very early in their marriage. That morning she had even told Heathcliff that she hated him, and Heathcliff says that he was thankful for that. (pg 148, chapter 14). Heathcliff says he doesn't care who knows it, their love was only one-sided, Isabella's. He has not shown her one sign of softness. The first thing he did when they left the Grange to get married was hang her dog!! (pg 148, chapter 14). She pleaded for its life, and he told her that he wished he could hang everyone close to her, except one. Of course, Isabella thought that one was herself, but it was Catherine. He tells Nelly that she can take a message to Edgar saying that he has never met such a contemptible person as his sister. He says that he will do nothing to cause their separation but....
"the nuisance of her presence outweighs the gratification to be derived from tormenting her" (pg 149, chapter 14)
Isabella replies that he is a liar. He told her she could leave him before this. She attempted it, and she won't do so again. She does not go into detail as to why.
When Isabella has had enough, she shows up at the Grange looking disheveled, with a deep cut under one ear, a white face scratched and bruised, and a "frame hardly able to support itself through fatigue" (pg 166, chapter 17) . She said she left him because...
"he was worked up to forget the fiendish prudence he boasted of, and proceeded to murderous violence."
After Catherine's death, he visited her grave continuously. One evening he came home early, and Earnshaw locked him out of the house. Earnshaw armed himself with a knife. Heathcliff broke through a window, grabbed the knife, and slashed Earnshaw up the arm, severing an artery. He held off Isabella with one hand to prevent her from summoning Joseph to help.
Finally the next morning, Heathcliff accused Isabella of conspiring against him with Earnshaw. "he shook me till my teeth rattled, and pitched me beside Joseph..." (og 174- chapter 17) He demanded that she deny any of his actions from the night before, and she tried to do that. Finally he tells her to leave the house. She said that she would remain to take care of Hindley. Heathcliff says, "Get up, wretched idiot, before I stamp you to death." (pg 177 - chapter 17). He snatches a dinner knife from the table and flings it at her head. It sticks just below her ear. She pulls out the knife and springs for the door. He attempts to follow, but Hindley blocks his path, and the last thing she sees is the two of them locked together on the hearth, fighting.
So, he was verbally and physically abusive to her. She feared for her life, so she left him.