Heathcliff is now acting as both father and teacher to Hareton. This assumption of the paternal role mirrors the way Hindley assumed Mr. Earnshaw's role as father to Heathcliff, and then immediately lessened Heathcliff's status at Wuthering Heights. Now, Heathcliff has taken both Hindley's father and son away from him. Hindley ended Heathcliff's formal education at Wuthering Heights; Heathcliff does the same to Hareton. This is the first part of Heathcliff's master plan for revenge.
After working his way back into Wuthering Heights, Heathcliff aims for Thrushcross Grange, directing the second part of his revenge towards Edgar by encouraging Isabella's infatuation. Heathcliff has no passion, love, or desire for Isabella; he only wants to use her. And he does. Having been so ceremoniously removed from Thrushcross Grange as a child, Heathcliff desires to acquire it, and Isabella supplies the means for him to do this.
Catherine's hysterics followed by her refusal to eat illustrate both her weakening mental and physical self. When she locks Edgar and Heathcliff together and throws the key into the fire, this is the height of romanticism for her. Symbolically, that key represents the key to her heart. She throws it away, and in doing so, actually pushes both men away. Edgar cannot understand her love for someone so crass and wild as Heathcliff; Heathcliff cannot fathom her attraction to the sniveling and weak Edgar. Curiously, Edgar is the one who acts out of character by striking Heathcliff. This action demonstrates the lengths that Edgar will go for the woman he loves.
