No two hens will be the same. Sometimes a hen won't even be the same twice. There may be no question if you SAW it happen or other behavior like the baby running from mom. I agree they do it sometimes even to their own chicks both with and without reason. But that doesn't instantly make every hen guilty.No question.
Besides... Dear OP, she should be working on integration with the flock if that's where those birds are going. The older they get, the less broody hormonal protective she will be, the more flock antics they face... Just a suggestion. I am very sorry your baby and you face this!