Okay, let's start with the fact that life starts going downhill for Wuthering Heights the moment he arrives, not just because Earnshaw dies soon afterwards. My theory is that Joseph is actually an immortal vampire who gains youth by feeding off of others' angst. Thus, when Hindley returns with a wife, Joseph realizes that he can't have the new master be happy- He kills Frances in childbirth, and plots to raise baby Hareton for his own uses (ie, making him miserable and using him as a food source). Unfortunately, Hindley spends all his time drinking, and Hareton is kept by Nellie- this won't do at all, so Joseph sets a dog on Catherine one day when she and Heathcliff are out too close to Thrushcross Grange. Also, to keep his angst sources high, he verbally abuses the kids. His mistreatment of Heathcliff allows the boy to turn into the rage-demon we all know and love, and marrying Catherine off to Edgar nets him a Hareton.
You know how the rest goes. Heathcliff becomes a neverending fountain of angst, and Hindley drinks himself to death. Unfortunately for Joseph, though, Hareton grows up to be insanely forgiving and inevitably turns on Joseph.
Joseph then flees to Mississippi, to annoy other students forced to read literary classics with his nearly unreadable dialect.
... Which is when I was banned from Arkham Asylum. Any questions?