But what else could she have done? Quantum's tracking her, so she can't just disappear. She can't seek refuge at MI6 because 1) They'd never accept her after discovering she's a double, and 2) Quantum has people inside MI6 (call that a retcon if you like). She can't run from them, she can't hide from them. If she did, they'd obviously kill her AND Bond. Bond wouldn't have taken too kindly to knowing about her being in league with Quantum had he found out before it became an endgame in Venice.White confirmed that if she hadn't killed herself, they'd have had Bond, too. Either he'd have to defect [of course not] or they'd have killed him. He didn't mind letting Bond go in Venice because Bond didn't have any info about Quantum. White didn't know about Vesper's text that revealed his cell #.
I don't follow that- if they wanted to kill him, how does Vesper killing herself stop them?
And 'when Quantum busted up Le Chiffre's party', she bargained with White to let Bond keep his life just by telling White that Bond had the money and she could get it without having to take it by force, presumably. When it's put like that you can see Bond's view on her as a 'bitch' as not being too far off the mark. Why not just tell Bond if she were under pressure from Quantum? If she actually were in love with him that would make more sense than stealing money from the government via him for some bloke that she doesn't love as much as she thought.
I've never been too happy with Vesper's motivations- it's all a little too cloudy and QoS doesn't help much.
Of course she's a bitch, especially from James' point of view. But he realized in the end that she was duped into working with Quantum and also sacrificed herself so that Quantum wouldn't try to hunt him down. That's why he's content by the end of QOS. Makes sense to me, anyway.