One chicken pecking vents

So my chickens are almost 9 months old months old, and just started laying a little over a month ago except for one (Choocher). She is also by far the lightest, but not sure if she is unhealthy or not. I haven't noticed the others keeping food from her, and In see her eat and drink.
About a week ago we got a really huge egg and then a few days later one of the chickens vents was bloody and her butt feathers plucked out. I separated her for about a week and got her looking great. I was just guessing maybe the large egg injured her and then someone went after her, but then it happened to another chicken and saw that Choocher's face was covered in blood. Both the injured chickens are doing great, but I have Choocher separated till I decide what to do with her...
She is the lightest, and not laying yet so I'm not if she needs more nutrition and what I should do about that or if she is just going to keep pecking the others bloody.

Any thoughts would be helpful. This is my first set of chickens first issue.
Thanks!

Do an on site on 'pin less peepers'. you may need to put these on her if she keeps up with this very dangerous habit. For sure you don't want the whole flock to copy this behavior.

What type of feed are they on? Not getting enough protein can cause cannibalism.

Do they have enough room in their coop/run/range?

I would bring this chicken into the garage in a dog crate and monitor her food and water intake. If she's just stuffing herself and NOT gaining weight back, I would consider worming her with Valbazen liquid goat wormer... especially if you see runny or bubbly poops.

You can get it at Tractor Supply, maybe your local feed store, and always online.

The dose is .5ml/cc for standard adult chickens, and .25ml/cc for bantams, pullets/cockerels. Give dose orally undiluted via syringe and then repeat in 10 days.

If she puts on weight in confinement, you'll know she was just not getting enough with the flock.

Hope that helps!

MrsB