If her tail is bloody now i cannot guarantee this but is there anyway a blood feather has been yanked? i had this a few times with a young group where the girls(sisters) we picking on each other, one would be bleeding and the other covered in blood at the bill face, they love to grab each other and will on occasion result in yanking those blood feathers.
Perhaps i have missed it but what does catching her and a bloody tail have to do with culling? some scovies are just a pest to catch, i would never cull a duck unless ill, sell her if she doesn't suit your needs, ducks are quite valuable, took me years to build my females up.