From chapter 11 until Catherine's death, Nelly and Catherine have a strained relationship, characterized by Nelly turning against Catherine. Nelly, by her own testimony, has been alarmed by Heathcliff's return. As Nelly explains at the end of chapter 10, her heart "cleaved to the master's," ie. to Edgar Linton. She doesn't trust Catherine's principles or emotions. She wants Heathcliff to go away, calling his presence "a continual nightmare." She feels "an evil beast [Heathcliff] prowled," and so she feels justified in interfering in events. She is angry at Catherine for encouraging Heathcliff.
After Edgar and Heathcliff have a confrontation, Catherine insists a "thousand smith's hammers are beating in my head!" She wants Nelly to tell Edgar that she, Catherine, is in danger of being "seriously ill." Nelly doesn't believe her and doesn't pass on the message. When Edgar visits Catherine soon after, and she passes out, Nelly whispers to Edgar that Catherine simply wants to manipulate him. As Catherine gets sicker and sicker over the next three days, Nelly still doesn't tell Edgar. When he finally sees Catherine, who by this time is losing her mind, he's shocked at "haggardness" of her appearance, he scolds Nelly for her withholding.
Nelly, angered that she is being blamed when she thinks the whole fault is Catherine's, tells Edgar that Heathcliff has been paying visits, at which point Catherine calls her a "traitor."
When Catherine is dying, Nelly, at Heathcliff's insistence, acts as a go-between, arranging for the two to meet one last time. At the same time, when Catherine once again faints after her visit from Heathcliff, Nelly thinks that it would be better for everyone "that she [Catherine] should be dead, than a lingering burden and a misery-maker."
Although Nelly does her best to justify her behavior, we wonder whether she didn't hasten Catherine's death by refusing to communicate how ill she had become. Her hostility to Catherine also leads readers to question if Catherine is quite the selfish, aggressive person Nelly makes her out to be. Nelly, through her own words, shows herself to be hard-hearted and lacking in compassion toward her mistress.