A few years ago two of my bantams went broody. Libby had gone broody first with Lucille going broody sometime later. They both sat on the same nest covering the same eggs. One day I had peaked in the coop and found a half-dead chick. I brought it in the house and tried to save it. Sadly it didn't make it. Sometime later, another chick hatched, same problem. Finally I caught Libby pecking at a chick that was hatching. I got her out as quickly as I could. She acted like she didn't even care! Thankfully, though, there was Lucille. She managed to hatch one more chick which was Lucy.
Lucille had another hatch of chick the next year.That time I had switched her eggs with standards'. That was the last time she has gone broody. Lucy has had two hatches herself. First was TSC Silkies and her latest was three of her very own. Those finished hatching yesterday.
Where is your hen in the pecking order? Some hens are bullies. Libby has always been at the top of the pecking order and she loves to attack and chase any hen that is bigger than her. I like to call her Queen Libby for she acts like the boss.
A hen attacking her chicks is not really that rare. Every time you have a new brooder you should watch it for the first few days. If your hen is known for killing frogs, mice, etc. chances are she'll attack chicks.
If you want an easy way to break her broodiness, try hanging a wire cage where she has no privacy and where she can't keep the underneath of herself warm ( I have never tried this, so I don't know if it works, but I have heard it works).
I had a hen who went broody just before I moved her to a new coop in a new flock. She had no access to her old nesting box. She tried everything to find a new nest which was including last years Christmas tree. She finally gave up, thankfully. She was a very skittish hen. Her name was Twilight. When she was younger she got bullied around some. I wouldn't trust a hen who has been bullied around chicks. I've seen it before where a bullied hen will become a bully. Has your hen ever been bullied by other hens?