| Quote #1
He [Hindley] has been blaming our father (how dared he?) for treating H. [Heathcliff] too liberally; and swears he will reduce him to his right place.(3.30)
Hindley started the whole revenge cycle by mistreating Heathcliff in the first place. His envy of Mr. Earnshaw's love for the orphan sets off a chain reaction of abuse and mistreatment.
| Quote #2
So, from the very beginning, he bred bad feeling in the house; and at Mrs. Earnshaw's death, which happened in less than two years after, the young master had learned to regard his father as an oppressor rather than a friend, and Heathcliff as a usurper of his parent's affections and his privileges; and he grew bitter with brooding over these injuries. (5.55)
Hindley's resentment has a very clear beginning. Before Heathcliff arrives, he is clearly the young man of the house, and he does not easily give up this privilege.
| Quote #3
[. . .] they forgot everything the minute they were together again: at least the minute they had contrived some naughty plan of revenge. (6.11)
Catherine helped make the misery more bearable for Heathcliff. She is not only his friend and sister, but his co-conspirator in revenge. Clearly Heathcliff was unwilling to sit back and accept poor treatment, even as a child.