Elle Driver: What's that?
Beatrix Kiddo: Budd's Hanzo sword.
Elle Driver: He said he pawned it.
Beatrix Kiddo: Well, then, I guess that makes him a liar, now don't it?
Bill: No. You're not a bad person. You're a terrific person. You're my favorite person, but every once in a while, you can be a real c**t.
Budd: That woman deserves her revenge...and we deserve to die.
Chosen answer: The way she talks to Budd as he lies dying, seems to indicate that she has disliked and hated him for years, plus the fact that she feels that he was not "worthy" of killing Kiddo (seeing how Kiddo was a fantastic warrior, while Budd is an alcoholic hick). However, it would be difficult for her to get close enough to kill him earlier, as he would not buy any excuse why she just came visiting. But when Budd actually invites her over, she gets a chance to get back at him, and in the same while take the credit for killing the Bride and retrieving her Hanzo sword for Bill. Vengeance and personal gain in one swoop.
Twotall