I think threatening to give away a pet is different than threatening to kill one. My mom did that (threatened to give away) a lot growing up, and once she did sell one of my horses. She said if I didn't work him everyday she'd sell him. Well, my friend ran me over with my go-kart and took all the skin off my shin from my knee to my ankle down to the bone (it was nasty, but not as painful as it sounds). After a couple weeks of not working him (because I couldn't ride him because my shin was so messed up) she sold him. I'm not gunna lie, I am still bitter about nearing 20 years later. But, at the same time I could've probably lunged him to keep him working. So, there is some blame on my part. Anyway, it upset me and I'm bitter about it, but if she'd have killed my horse that would've been an emotional scar that would've been beyond horrifying.
I'm sorry your mom threatened to flush your bettas. At 59, I would think she would be through menopausal swings. My moms 10 years younger though, so I'm not sure. Good luck with your situation.
The mom of: Too many to list now.