I can believe a cot can hold on to a parakeet long enough to kill it - they are much bigger and stronger than buttons.
You have 5 quail you mean, right?
As to why she does it.. I actually have 4 females and one male cot in an aviary that's right about 7x8 ft. And guess what? One of the females is being aggressive. She had pecked the male so much there was blood on his shoulders about a month ago. I put her in a guinea pig cage inside the aviary for a couple of weeks to cool her down, then I let her back out again. She was nice at first, but I saw her chasing and pecking one of the other females again yesterday.. I might try to put her and the guinea pig cage outside the aviary if she keeps it up, so I break up her relation to the flock. Or I might just cull her - and the male as well, he's not very nice either - when my first batch of chicks is ready to replace them/go to the freezer in a few weeks.
I think your hen - and mine - are just particularly territorial birds that don't tolerate the conditions we provide them with very well. Although we both provide them with seemingly good conditions, we are still very far from how they'd live in the wild and some birds just don't cope very well with that. The yellow parakeet was probably singled out because it looked 'different' - quail can be racists towards other colors of their own species as well.
If you are lucky, it's something hormonal that will pass soon, but unless you are giving them additional light they should be less aggressive in the fall and winter than the rest of the year. Mine have no additional light, yet they are still laying and the hen is still aggressive with just 12 hours of light, so I don't expect that to pass..