Molly didn't had the strength to kill Bellatrix... She never had it. But she did manage to do something to the witch, she took something from her. Something that meant almost everything to Bellatrix and also almost everything to us.

'That's odd...' she could tell with every inches of her frame that something was off. The place where she was, yes ti was the same place as the one of before where all that had happened, but at the same time it was different... It was almost as a painting?