I understand completely, my mare stomped a young pullet one night that was underfoot, and she meant to stomp it, killed the pullet dead. I did fuss at the darn horse, but she was just very annoyed at chickens being underfoot, her way of saying so. I think your little sheep was "playing" with the fowl, like a dog would. It's fun to chase around a silly chicken. Sorry you lost a little friend. Grow that nice lamb and have BBQ.