In the end, it is the fact that Catherine never really loves him which drives Heathcliff mad, making him do the vile things he does. As a rule, madness seems to have come fairly easily to the English countryside. Catherine (elder) goes mad, Heathcliff goes mad, and both Earnshaws are totally loony. What I can't decide is whether or not people in Victorian England were actually that crazy or if it was just a convenient literary trope. It certainly seems to do the job in the novel, since it comes on with little or no provocation and usually results in death, often unexplained. Paced out differently, that's pretty much how all of the above characters go. Perhaps that was believable two-hundred years ago, but it hasn't helped the book age well in my opinion.
At the end of things, I think this book is a worthwhile read, but not truly compelling. While there is a fascinating level of complexity, and it certainly bucks the trend of other Victorian novels I've read (which, I'll be the first to admit, is not many), there are simply too many unanswered questions. Too much of Heatcliff's life and thoughts goes unrevealed. The book is his book, and I think the reader has a right to understand more about him by the end. Much of this limitation comes from the first person narration through Nelly. Initially, I thought it was a great idea, but as the novel progressed and my questions continued to endure, I grew irritated with both narrator and author. It is a fine novel, and worth a read when you are searching for something against the grain, but I am going to stop short of using another five letter word, "great," with regard to Emily Bronte's Wuthering Heights.