we had a similar thing happen a couple months ago. Frankie killed my baby chicken, we had her in the back birds room until she was old enough to put out with the other hens and rooster. I laid down to take a nap and about an hour later the conure and cockatoo started screaming, when I got up a few mins later found him outside with the baby chicken deceased. Its a "natural" thing, Greed, Jealousy, wanting to be protective of the ones they love, anything. Frankie probably did it out of jealousy because I was spending so much time with them but you really can't be mad at them for it. They don't understand why its wrong, and you can't really punish him for it. Just do evreything in your power to PREVENT it.