I gave my bad mom hens to a neighbor down the road, along with an extra roo. He only has one other hen. It's possible that without all the commotion of my flock, (36 adult/and near adult chickens, around 30 chicks, a poult, 10 adult guineas and 8 keets) multiple broodies at once, young roos running around, etc., they may brood and hatch and mother just fine, in a quieter environment. Or not, but they got a new home, anyway. I considered eating them.
I have a theory. Layer breeds are breed to discourage broodiness, and increase egg production. Hatcheries breed production layers into other breeds, from time to time, (which is why serious breeders don't generally want hatchery stock) to increase egg production so they can have more eggs to hatch/sell. I think this mixing of non-broody breeds with broody breeds, can sometimes mess up their instincts, like my two bad moms, and your killer mom. They get parts of the brooding/mothering instinct, other parts are missing.
I guess I've been lucky not to encounter such a thing as what you just went through.
I have a theory. Layer breeds are breed to discourage broodiness, and increase egg production. Hatcheries breed production layers into other breeds, from time to time, (which is why serious breeders don't generally want hatchery stock) to increase egg production so they can have more eggs to hatch/sell. I think this mixing of non-broody breeds with broody breeds, can sometimes mess up their instincts, like my two bad moms, and your killer mom. They get parts of the brooding/mothering instinct, other parts are missing.
I guess I've been lucky not to encounter such a thing as what you just went through.