If it were me.... I would keep him separate and experiment with his diet to see if you can curb his aggression. I have a few pionus, but I mostly deal with aggressive 'toos. First thing I will admit is that pionus here never get pellets... they get fresh food and a limited amount of seed. I would add tofu to his diet and cut out seed altogether for a while. If that didn't help I.would put him on avicalm while I decided what to do. The truth is if I ever honestly believed a bird was a mate killer it would not leave here.... ever. I would either retire it here alone or have it euthanized... but I am very picky about what I use for breedi g stock and will only use what I believe best represents a breed in size, color, health, and disposition. (I mean when I decided I wanted greenwing macaws I couldn't find adults I liked, so I bought unrelated handfed babies that were the size of hyacinths with color to kill for.... good stock is always worth the wait and we want to leave aviculture better off than it was when we started. If it turns out not to be diet, I would be sure never to pair him, and never to let him out into mainstream aviculture and risk hens in the future. (Just my opinion)