I think the root cause of his revenge should be his dissatisfaction with racial discrimination. He fell in love with Catherine because Catherine didn't care about the fact that he was a gypsy. There is a scene in the film where Gil-galad brothers and sisters insult him. So the fundamental reason for his revenge should be the pursuit of equality. As for the fundamental reason why he can't marry Catherine, it is also because he is not a rich gentleman. This can also be seen from his attitude towards Nelly, who is a maid of Earnshaw's family and a person of lower status. Therefore, Heathcliff has a kind of respect for Nelly, who is in the same boat.