I totally disagree about Mrs. Weasley's line. Her use of profanity indicates not that she's being protective, but rather that she's out of control of her emotions. This is a fantasy book, and this single word pulls it back into the world of reality --ugly reality, at that. It stands in stark contrast to other fight scenes in which the dialogue has been often sarcastic or witty, and sometimes even mean-spirited, but never ugly. The emotion of the scene -- a woman who's lost one child, had another child scarred for life, and knows that her other children and her husband could still be lost at any moment -- could've been portrayed with class and without resorting to gutter language. It befouls a character who has always been the archetype of motherhood and patience; she doesn't look strong -- she looks desperate.
I think you're right about JKR not wanting Neville to kill -- it is in keeping with his character, though I still think it'd have been poetic justice for him to "do in" the woman who tortured his parents and essentially took away his childhood.
