Some readers may sympathize with him because he was abused. We rationalize his behavior due to the fact he endured so much abuse.
Heathcliff is such a diverse character in this novel that his character is able to repel and engage us. He repels us in so many ways. One way he repels us is how he treats the other characters in the book. Isabelle was a prime example of this. Heathcliff actually used Isabelle as a tool to get revenge on the Linton family. Isabelle fell in love with him and he used her for her whole life and never once returned that love to her. This is only one reason why Heathcliff repels readers from liking him. I think that Bronte made this stick out in the novel for a reason. Isabelle was abused by Heathcliff and she kept going back for more because she had hope in Heathcliff that he had a little bit of good in him. We, as readers, do the same thing that Isabelle did. Instead of crawling back to Heathcliff, we kept reading the book. Bronte made this extremely clear as I continued to analyze the text. Heathcliff also treated other characters poorly. He hung Isabelle's dog, he essentially tried to steal Edgar's property, and so much more. I believe he does all of this to get back at Edgar for marrying Cathy and among other things he despises the Linton family for. .
Now, even though there was multiple things against Heathcliff, there is one huge reason why readers love him the most, the apparent love he had for Catherine. He made himself look crazy for her and her only. He loved her beyond any care he had for people and what they thought of him. There was one problem that sticks out to me. The problem of the bond between Cathy and Heathcliff and its significance was that it remained the central mystery of the novel till the very end. That also helped the readers keep intrigued till the very end and keep that hope that I have previously talked about. I believe that their love existed on a higher or spiritual plane; they were soul mates, two people who have an affinity for each other which draws them together irresistibly.
