Do you understand his character at all? Neville is not a hated filled person who’s only aim is revenge. He knows there is a higher cause to be won rather than satisfying his own ego. He would never stoop to Bellatrix’s level and that’s probably why Jo didn’t have him kill her. Plus, you cannot tell me you didn’t love the line “NOT MY DAUGHTER YOU BITCH!”
Killing Nagini was much more important- he finally got his chance to shine and helped with the bigger picture- making Voldemort weaker. Plus, I think Molly killing Bellatrix is awesome because it shows the difference between maternal and obsessive love and what a bad ass Molly Weasley is. I think J.K Rowling captured it really well.
As she said (along the lines) ‘Not sure if there’ll be tomorrow.’ Her children would have been left orphaned and who would take care of them? I mean, Molly’s brothers died and Arthur hasn’t mentioned anyone he’s close with.