How does heathcliff save hareton's life?

Basically, in chapter 9 of Wuthering Heights, Hindley comes home in "a drunken rage" and, due to the loss of his wife, becomes numb to most things in life. While Nelly is cleaning, (and attempts to hide baby Hareton from his father), Hindely catches her, and orders her to hand him over. She relucantly does, and being the douche Hindley is, he holds Hareton (his son) over the banister of the stairs and proclaims that he wants to "smash the brats skull" in. He lets go of his son, who falls, but, in a weird turn of evens where Heathcliff is actually nice for one of the only times in the novel, he runs underneath Hareton, and saves him from literally smashing his skull in.

It is also important to note that as Heathcliff is holding hareton in his arms after saving his life, he looks up and realises that if he wouldn't have let his instincts (y'know, those instincts everybody has to catch a baby if it's falling from a 12-foot staircase) drive him, it would have been the perfect opportunity to get back at Hindley for being a dick to him as a child - letting his own child die at the hands of his father.

Legit.

This actually happens.

Heathcliff is so pissed with himself for saving a baby that otherwise would have broken almost every bone in its body and died instantly from the impact.

He's actually pissed about not stepping out of the way and letting the child die.

*sighs*

Heathcliff, why u so mad?