How does Heathcliff unwittingly thwart his own revenge? What poot behavior on the part of Hindley leads to the occurrence?

Hindley becomes verbally and physically abusive to Ellen. Hearing Heathcliff walking below, Hindley accidentally drops the child, but fortunately Heathcliff catches him. Looking up to see what had happened, he showed "the intensest anguish at having made himself the instrument of thwarting his own revenge" (75). In other words, he hated Hindley so much that he would have liked to have him to kill his own son by mistake. If it had been dark, Ellen said, "he would have tried to remedy the mistake by smashing Hareton's skull on the steps." Hindley was somewhat shaken, and began to drink more. Heathcliff told Nelly he wished Hindley would drink himself to death, but that was unlikely to happen as he had a strong constitution.