I use to keep.... well allowed a corn snake to stay in my feed room. The problem was that i discovered him too often in places and under thinw gs that i wasen'
Don't confuse plain black snakes with king snakes. King snakes kill and eat all North American species of poisonous snakes. And yes a king snake will also eat eggs and chicks.
BTW, king snakes make pets at least as good as chickens do.