To quote the Beastie Boys, "She's crafty." Arabella knows what she needs to get by, and she does what she needs to do to get it.
Arabella basically tricks Jude into marrying her twice. She also drops a child in his lap who ends up killing his other children, which leads to the end of his life with Sue. (Well, to be fair, Arabella couldn't have predicted LFT's extreme reaction to Jude and Sue's poverty. But Arabella does leave a deeply emotional sensitive child in the care of people who can't take care of him financially—not a great move.)
Arabella shows up throughout the novel just when it looks like Jude and Sue might have actually found some happiness. It's also no coincidence that Arabella is the first thing that throws Jude off course in relation to reaching his goals by going to Christminster. The final acts of Arabella are what truly sum her up. She finds her husband Jude dead, and steps out to go see a boat race. That's cold-blooded. However, to her credit, Arabella manages to survive in a difficult world. As she says, 'Weak women must provide for a rainy day' (6.10.31).