Nelly Dean is the primary narrator in "Wuthering Heights."
She is NOT the villain of the novel.
Although she is the primary narrator, she is also a character who takes part in the action in the novel.
1. In Ch.4 Nelly tells us how she joined Hindley in hating Heathcliff: "and to say the truth I did the same."
2. In Ch.9 Cathy confesses to Nelly that she is in love with Heathcliff and Linton at the same time. Nelly refuses to promise to keep this a secret. This is why she reveals this to the secondary narrator Lockwood and so we the readers are able to understand Cathy's complex relationship with both Linton and Heathcliff.
3. Ch.12 clearly reveals the dilemma Nelly finds herself in. Cathy and Linton quarrel and Cathy locks herself up in her room while Linton ignores her and immerses himself in his books. Cathy becomes dangerously ill but Nelly thinks that nothing is seriously wrong with her and does not inform Linton. When Linton comes to know of Cathy's precarious state of health he becomes angry and scolds Nellly to which she replies "I performed the duty of a faithful servant in telling you, and I have got a faithful servant's wages."
4. In Ch24. younger Cathy tels Nelly Dean how she made fun of Hareton Earnshaw's illiteracy. At once Nelly Dean checks her by saying that Hareton is her cousin and that she should not make fun of him, thus sowing the seeds of love between Younger Cathy and Hareton.
It is really difficult to pinpoint whether Nelly is the villain of the story and if she is to blame for what happens in Wuthering Heights. It is difficult to tell because Nelly is first and foremost the main narrator of the story. Even though she is delivering the story it is more than easy to construe the story to make it look yourself look harmless and a self mediator. There are many parts in the story where she sort of boasted about herself being kind and caring but throughout so much of the story she does nothing but judge everyone. One moment where she boast about herself to make it seem like she is really caring is when Hareton is took from her care she says a couple of times that she didn't want to let him go and she cared a lot for him. Also there is the time were she talked about begging Edgar putting little Cathy in the will for the estate, but in the same breath she vilifies Heathcliff for morning Catherine's death the he did. So again it isn't difficult for her to make herself try and seem more decent than she may be since the story is heavily relied on her recount. It's highly possible she could have left out crucial parts where she was involved at all.
Nelly is COMPLETELY the villain of the piece and an archetypal example of the unreliable narrator. She is a self-serving gossip whose resentment of her social status causes her fan the flames of the conflicts throughout the novel. In Chapter 4, she puts Heathcliff, th whom she refers as "it", out on the landing of the stairs, hoping it will be gone by morning. She says to Lockwood, "my pinches moved him only to draw in breath and open his eyes, as if he had hurt himself on accident and nobody was to blame." When Cathy is a wild child, she scolds her. When Cathy is a lady, she resents her ambitions for overshadowing her own. She villifys Heathcliff while simultaneously egging him on. The little examples go on. Read the novel while keeping in mind that Nelly always tries to portray herself in the best possible light, and becomes strangely silent on matters where she cannot, and the examples become obvious.
Google James Hafley's essay "The Villain in Wuthering Heights". A PDF of it should be the second link down in the list. I believe you'll find it makes a very compelling case for Nelly as the villain. As with other unreliable narrators, what she does not tell us is often as instructive as what she does, a subtlety lost on the innocent Lockwood, and apparently most everyone who has read the book for the last century and a half.