Neville should have been the one to kill Bellatrix

So I know that the whole brand of Harry Potter is that the protagonists don't kill, that being the reason why Harry uses Expelliarmus so much- it's self defense, rather than attack. But somehow, Molly killed Bellatrix without using the killing curse, so considering it's possible, Neville should have been the one to kill her. I know that the "Not my daughter, you bitch!" line from Molly was incredibly iconic, but Neville not doing ANYTHING to Bellatrix wasn't really satisfying. Again, I know that was sort of the point; to not have him retalliate, but WHYY?