In the books, she tries to kill Ginny, then taunts Molly Weasley about Fred dying and how Ginny is going to go the same way. Molly's blind fury causes her to kill Bellatrix (and to shout, "not my daughter, you *****!") - ordinarily she wouldn't be any match for Bella but in this case, she's avenging her child's death which gives her the edge.
It was a great part of the book actually, because Voldemort witnesses it and is furious, tries to kill Molly and that's when Harry puts a shield charm round Molly, reveals himself and shows that he survived the killing curse again, leading to their final battle.
Ahh that sounds much better. I understand they have to edit a lot in order to get the run time down but that sounds like a scene worth doing! Shame.
