I have a friend who owns and breeds chickens---she buys chicks, but has to keep them in a smaller cage inside the roosters and chickens' enclosure, because the older birds attack them. We don't know if they'd kill them, but they certainly peck at them---she raises them separately until adulthood, and then lets them loose with the others. However, chicks born of a hen by one of the roosters will be fine; they won't get attacked.