Miss Dartle is living proof to us (if we need more evidence) that Steerforth's fatal flaw is his uncompromising, stubborn temper. Miss Dartle is an orphaned cousin of Steerforth's late father. Mrs. Steerforth brought Miss Dartle to live with her as a companion once Mr. Steerforth died.
One day, Miss Dartle and young Steerforth fall to arguing. Steerforth is so furious that he throws a hammer at Miss Dartle's face. Even though Steerforth feels bad about it later, he can't take away that moment of temper that has left Miss Dartle forever changed. She bears the scar on her lip to this day.
Miss Dartle's scar is a sign of her own marked, twisted nature: interacting with the Steerforth family has really left her a piece of work. She absolutely loves Steerforth – when he dies, she shouts that, "He had a soul worth millions of the friends to whom he stooped!" (56.54). At the same time, Steerforth's presence seems to taunt and irritate her.
David notices immediately, when he first meets Miss Dartle, that she has this habit of taking pretty much everything Steerforth says and twisting it to show how stupid he can be. The best example of Miss Dartle's particular sarcasm is when Steerforth mentions in passing that he thinks the poor are not sensitive, and that they don't feel pain the same way the upper classes do. Miss Dartle replies:
"It's such a delight to know that, when [the poor] suffer, they don't feel! Sometimes I have been quite uneasy for that sort of people; but now I shall just dismiss the idea of them, altogether. Live and learn." (20.40)
Miss Dartle is really laying on the sarcasm here, by suggesting that she feels "delight" that poor people "suffer" but don't "feel" – which is clearly an impossible contradiction. Miss Dartle is quick to jab at Steerforth when he says things that are stupid or offensive. And Steerforth agrees later that Miss Dartle is "all edge" (20.43) – she is never soft with anyone, not even Steerforth.
Despite her frustration with Steerforth, though, she is painfully jealous and loyal to him: it is Miss Dartle who tracks down Littimer and finds out the story of Emily up until Steerforth deserts her. And it is Miss Dartle who finds Emily in London even before David can get there.
Once Miss Dartle arrives at Emily's room, she immediately starts to spew awful things at Emily, implying that Emily deserves to die for being so awful as to seduce a great man like Steerforth. All of the potential sympathy for the poor that Miss Dartle expresses in her sarcastic jab above seems to wither away when confronted with a romantic rival for Steerforth's love.
When Steerforth dies, the crazy conflict between love and hatred at the heart of Miss Dartle's nature comes spilling out. She screams that it was Mrs. Steerforth who trained him "from his cradle to be what he was, and stunted what he should have been" (56.43). Miss Dartle cries out that she is the one who truly loves Steerforth – as patiently and self-sacrificingly as anyone in the book – but she was prevented from expressing this love because of what Mrs. Steerforth made him.
Mrs. Steerforth encouraged Steerforth to be vain and "inconstant" (unfaithful). While Steerforth seemed to return Miss Dartle's feelings when he was a boy, he grew tired of her and cast her aside. After this, Miss Dartle became "a mere disfigured piece of furniture between [them] both – having no eyes, no ears, no feelings, no remembrances" (56.59).
In other words, once the Steerforths began to take her for granted, they both lost any sense that Miss Dartle is a living being with her own thoughts and feelings. She was only a real person to them while they remained interested in her, and once they both lost interest, she became about as an important as a chair: handy, but not exactly an object of much concern.
Miss Dartle is twisted and horrible. She deliberately seeks out Emily at her most vulnerable to insult and threaten her. And even though she takes care of Mrs. Steerforth in her old age, she spends a lot of time mocking and reproaching her for spoiling her now deceased son.
Still, we cannot imagine the true horror of this woman's life. She is desperately, honestly, whole-heartedly in love with what Steerforth might have been – and perhaps was as a boy. But she has had to live with Steerforth actually as he actually is: vain, selfish, thoughtless, careless, and unfaithful. And now that Steerforth has died, Miss Dartle has to live with the woman who made him that way, and who is also the incarnation of all of Steerforth's own worst qualities.
It's as though Miss Dartle has to live her life constantly rubbing against sandpaper: she is never allowed to forget her own disappointment and misery. With this kind of constant agitation and irritation, it's no surprise that this clever woman turns into a vengeful shrew.