Feather pecking

Please can someone help me I'm at a loose end as what to do. I have 6 silkie bantams 3 black and 3 white. My once docile White hen Millie has suddenly turned very violent towards my black hens. I think it's more than just feather pecking, she growels when they go near her or if anything goes near her. Suddenly she puffs right up and violently attacks the black hens biting them and growling at them. It seems too violent for normal feather pecking as I've had that before. I bought some anti peck spray which made no difference at all, infact she seemed to like the taste and made her worse. Within 15 minuets of applying it two of my hens were down on their sides and were unable to balance so I rushed in and bathed them left them indoors to dry and they seemed fine afterwards. The hen I have separated into a cage and I'm worried about putting her back with the others. Can anyone please help I'm at my wits end and worried thanks