Whew. Arabella is a piece of work. She is so selfish, thoughtless, and manipulative that it's hard to take her seriously as a character in some ways—she's like a cartoon villain or something. Like, if this were a romantic comedy, she'd be Rachael McAdams from Mean Girls. Of course, this isn't a romantic comedy (again, it doesn't get much less funny than Jude the Obscure), so Arabella's behavior gets a lot worse than backstabbing and prom antics.
Arabella tricks Jude into marrying her…twice. In fact, it is almost impossible at times to know when—if ever—Arabella is telling the truth. After all, this is a woman who fakes the dimples on her cheeks:
As the girl drew nearer to it, she gave, without Jude perceiving it, an adroit little suck to the interior of each of her cheeks in succession, by which […] she brought as by magic upon its smooth rotund surface a perfect dimple. (1.6.35)
How are we supposed to trust someone who deliberately lures Jude in with false dimples?
In the end, Arabella plays an enormous role in Jude's and Sue's downfall. She cares nothing about the consequences of her actions, and she cares about no one other than herself. As Jude lies dying, Arabella is already hitting on another man to take his place, and when she finds Jude dead she just leaves him there so she can go to a boat race.
Still, we have to give Arabella credit for one thing: all of her actions in the novel are despicable and deceitful. But she does understand people and the way the world works about a thousand times better than Jude and Sue do. As Jude and Sue crumble, Arabella is just the same as she ever was at the end of the novel. Hardy uses Arabella to suggest that social conventions can destroy idealistic, good people like Jude and Sue—but they can positively benefit hateful schemers like Arabella.
Keep in mind that Hardy gives her the last line of the book. And she uses her last word to take one last bit of glee in Sue's misery while at Jude's funeral. Talk about cold-blooded:
'She's never found peace since she left his arms, and never will again till she's as he is now!' (6.11.74).Arabella Donn Timeline