I am so sorry to read that you are dealing with such an horrific situation.
Some chickens idea of "enough space" is very different to others and can be significantly different to some poultry keepers and coop manufacturers too...... we all know of the coops that are advertised for up to 6 chickens when you wouldn't put more than 2 in them. Leghorns are flighty birds that prefer more space. If they don't have access to food all day and have nothing else to occupy them, they will get bored more easily that larger breeds and that is when problems like this occur. Also, you say that you mix crumble and scratch. This may be another part of your problem. Birds will eat scratch rather than their regular feed, just like children will leave their dinner and eat dessert. Unfortunately scratch is lower in protein, so the higher ranking birds (your leghorns I would imagine) will probably eat most of the scratch and then be lacking in enough protein in their diet. That, coupled with frustration at having nothing else to do all day and maybe a weaker member of the flock becoming easy pray, leads to this sort of horrible behaviour. I think @chickengeorgeto makes a good point although I'm not convinced it will be quite as high as 99%. I have certainly seen my leghorns change from happy flock members when free ranging to ill tempered bullies when penned up.
Anyway, I wish you luck with nursing your girl back to health. Chickens can be amazingly robust sometimes.... I hope this is one of them. Also good luck rehoming your leghorns. If you can find someone who is going to free range them, they will be happier.
Best wishes
Barbara
Some chickens idea of "enough space" is very different to others and can be significantly different to some poultry keepers and coop manufacturers too...... we all know of the coops that are advertised for up to 6 chickens when you wouldn't put more than 2 in them. Leghorns are flighty birds that prefer more space. If they don't have access to food all day and have nothing else to occupy them, they will get bored more easily that larger breeds and that is when problems like this occur. Also, you say that you mix crumble and scratch. This may be another part of your problem. Birds will eat scratch rather than their regular feed, just like children will leave their dinner and eat dessert. Unfortunately scratch is lower in protein, so the higher ranking birds (your leghorns I would imagine) will probably eat most of the scratch and then be lacking in enough protein in their diet. That, coupled with frustration at having nothing else to do all day and maybe a weaker member of the flock becoming easy pray, leads to this sort of horrible behaviour. I think @chickengeorgeto makes a good point although I'm not convinced it will be quite as high as 99%. I have certainly seen my leghorns change from happy flock members when free ranging to ill tempered bullies when penned up.
Anyway, I wish you luck with nursing your girl back to health. Chickens can be amazingly robust sometimes.... I hope this is one of them. Also good luck rehoming your leghorns. If you can find someone who is going to free range them, they will be happier.
Best wishes
Barbara
She may be smaller than everyone else but I see where Leghorns do need added space. 
