Jw how old were the chicks? And did you see her eat them? Cuz I had similar thing happened to me. Chicks were barely hatched day one I went to check and found a dead headless chick day old only. Only one though nothing ever happened to the rest out of ten chicks. I assumed the chick died prob from many reasons and maybe a mouse got to it. Interesting if it was the hen as this never happened to me before til now.
I am guessing her cage with the babies is too small. Some new mother hens stress hard when they have babies. Give her a larger pen and create an activity for her, such as grass clippings, leaves, manure or compost to scratch in. She will be busy and not fixate on the chicks and freak out. She may never do it again.
They where four chicks altogether the two she killed were blue the other two where not and a couple of days before she tried to kill one of the blue chicks she kept trying to peck him but I managed to save him I should of realised then what she was going to do
Had hen do same thing hen had 4 chicks in a 12 by 5 pen the chicks were a week old found one dead with its head striped and blood every wear but I thought it was vermin getting to them I then went in day after saw another one the same but was still alive but had to cull. I sat and watched her through a tiny hole for over an hour then she started doing it to another chick so I quickly got in and dispatched her.the hen had fresh water food everyday was also getting worms and insects from the ground so dout it was down to nutrition