"Have you considered how you'll bear the separation, and how he'll bear to be quite deserted in the world?" (9.98)
Catherine can hardly think beyond her own desires. Nelly makes a futile attempt to defend Heathcliff here. She knows that if Catherine marries Edgar, it isn't going to be pretty.
"What is it to you?" he growled. "I have a right to kiss her, if she chooses, and you have no right to object. I am not your husband: you needn't be jealous of me!" (11.45)
Just because Heathcliff's not her husband doesn't mean Catherine won't act jealous and possessive of him. In this moment, it's hard not to be on Heathcliff's side.
"I want you to be aware that I know you have treated me infernally—infernally! Do you hear? And if you flatter yourself that I don't perceive it, you are a fool; and if you think I can be consoled by sweet words, you are an idiot." (11.49)
Though he knows he has been cruelly treated, Heathcliff cannot help but love Catherine. As readers we have waited for the moment that Heathcliff gets in Catherine's face about her behavior.
Ellen "Nelly" Dean
There was another rapid glance at the house, and supposing himself unseen, the scoundrel had the impudence to embrace her.
"Judas! Traitor!" I ejaculated. "You are a hypocrite, too, are you? A deliberate deceiver."
"Who is, Nelly?" said Catherine's voice at my elbow: I had been over-intent on watching the pair outside to mark her entrance.
"Your worthless friend!" I answered, warmly, "the sneaking rascal yonder. Ah, he has caught a glimpse of us—he is coming in! I wonder will he have the heart to find a plausible excuse for making love to Miss, when he told you he hated her?" (11.36-39)
Heathcliff's plot against Edgar begins, and Isabella becomes his willing dupe. Nelly doesn't exactly maintain calm in the situation. Look at her language—she clearly wants to provoke Catherine. And who exactly is the other hypocrite to which she refers here?
"She abandoned [her home] under a delusion," he answered, "picturing in me a hero of romance, and expecting unlimited indulgences from my chivalrous devotion." (14.35)
Like many others in the story, Isabella is influenced by all the novels she reads. She has certainly changed her opinion of Heathcliff from when they were children.
And then I remembered Mr. Edgar's stern rebuke of my carrying tales; and I tried to smooth away all disquietude on the subject, by affirming, with frequent iteration, that that betrayal of trust, if it merited so harsh an appellation, should be the last. (14.47)
Nelly tries to abide by her master's dictates, but her sympathy to lovers is greater than perhaps even she will admit. Again, why would she continue to go to the Heights and introduce Heathcliff to young Catherine?
"You teach me now how cruel you've been—cruel and false. Why did you despise me? Why did you betray your own heart, Cathy? I have not one word of comfort. You deserve this. You have killed yourself. Yes, you may kiss me, and cry; and wring out my kisses and tears: they'll blight you—they'll damn you. You loved me—then what right had you to leave me? What right—answer me—for the poor fancy you felt for Linton? Because misery and degradation, and death, and nothing that God or Satan could inflict would have parted us, you, of your own will, did it. I have not broken your heart—you have broken it; and in breaking it, you have broken mine. So much the worse for me that I am strong. Do I want to live? What kind of living will it be when you—oh, God! would you like to live with your soul in the grave?" (15.37)
In other words, you have no one to blame but yourself. But Catherine never really sees it that way. And though she continues to love Heathcliff, running away with him is never an option—nor does he ask her to. Why?
Ellen "Nelly" Dean
"The harm of it is, that her father [Edgar] would hate me if he found I suffered [Cathy] to enter your house; and I am convinced you have a bad design in encouraging her to do so." (21.36)
Nelly clues in to Heathcliff's ill intent, but that doesn't stop her from being suckered into going back to the Heights.
Mr. Linton was alarmed and distressed, more than he would acknowledge to me. In the morning, Catherine learnt my betrayal of her confidence, and she learnt also that her secret visits were to end. (24.84)
It is never entirely clear where Nelly's loyalties and intentions reside. Is she afraid to say no to Catherine? Does she actually enjoy the drama up at the Heights? Think about it!
"Oh!" he sobbed, "I cannot bear it! Catherine, Catherine, I'm a traitor, too, and I dare not tell you! But leave me, and I shall be killed! Dear Catherine, my life is in your hands: and you have said you loved me, and if you did, it wouldn't harm you. You'll not go, then? kind, sweet, good Catherine! And perhaps you will consent—and he'll let me die with you!" (27.16)
Linton Heathcliff can no longer hide his profound fear of his own father. The worst part is that he wants Catherine to sacrifice herself for him because he is too much of a wuss.