I remember my father telling me that a weasel will go for the throat and consume the blood... I know that this sounds like something from Grimms Fairy tales but there is often something to "wive's tales" or in this case "father's tales's"
I think I would advise setting traps and remove as you see fit. Whatever it is has had success and will be back. I'm sorry for your loss. I try not to get attached to my hens, but can't seem to help myself.
Are the roosters and the hens the center picture pairs? What is the color, and how did you get it? I have some mixes that I'm trying to figure out, and one is a similar color to the buffish hens with the blue skirts.