Wuthering Heights may be perceived to be just another novel, waiting to be read by the average reader, sitting on some remote bookshelf. This narrow-mindedness will be out of the mind of the reader when he turns past the first page; this novel is anything but conventional. Some characters have more depth to us than many real-life people, and one such character is Heathcliff. Ravaged by the past and bent on avenging everyone who mistreated him (and their loved ones), he is called insane by many. That first impression would be true if the character was taken, placed alone, and analyzed. "Insane" would be the result, unless the character is analyzed properly - against the backdrop of his past, his surroundings, and his loved ones.
Considering Heathcliff's past, he has almost every right to be insane. He was abused - after Mr Earnshaw died, he was no longer treated as an equal part of the family.
He became a servant - working on the field all day, not having access to education, and so on. This greatly scarred him; those actions on behalf of Hindley and the Lintons are responsible for Heathcliff's emotional state and his grudge against everyone but Catherine. This grudge - is responsible for many major events, such as the capture and "imprisonment" of Cathy and Nelly, and Heathcliff's decision to force the marriage between Cathy and Linton. That shows that the people surrounding Heathcliff are to be blamed for his actions.
The reasons for Heathcliff's torment don't stop there. Quite probably the most important reason is Catherine - or rather, the absence of her. His whole life prior to Catherine's death, he spent thinking of her. Their love was more than the love of any other couple - it was on a...