Since a lot of people questioned or disputed the credibility of Mrs Weasley killing Bellatrix, I will try to play the devil’s advocate and put it a bit in to reality contex.
First, Bella was so full with her self and underestimate the little housewife Mrs Wealsey.
Second, Mrs Weasley here resembles the natural instinct of mother (parents) defending her/their youth, under such circumstances people are known to perform extraordinary things, what under other circumstances are impossible. Bella threatened to kill her children.
Thirdly ” He is not your son Molly” ” Yes, but he is as good as one”
She believed that Harry Potter was killed. Some one she liked like her own son. Some one she owned the lives of her daughter Ginny, her husband Arthur. Killed by the group of Bellatrix
Last. Her son was killed, one of the twins, she turned in to a lioness, kill…kill…kill….
What was the motivation of Bellatrix… she was happy Voldermort killed Harry she believed, so she underestimated the little housewife…. she beleived the success was so near, and she became reckless... And she died! and I'm happy for that