Hmmmmm, this discussion has come up many a time. True, there are some cats who won't hurt a chicken, full grown or not. But some do, and you simply can not trust an unknown cat with your chickens. i have tried to be accommodating to neighboring cats. One beautiful persian, who belonged to a neighbor, liked to hang in our yard. Thought it was harmless, until one early evening when he snuck around behind me and jumped out to kill a full grown chicken right in front of me. Thankfully i was right there and screamed so loud he ran. i got the hose and squirted him, then followed him to the back of the yard squirting until he was over the fence, then i kept squirting. He hasn't been back. But that was the last time i thought some pretty little neighbor kitty was benign.
So i guess i take issue with all the folks that post about how safe it is to let cats romp with your chickens, and that one peck from the chicken and the cat backs off. Just because some people have not lost a chicken to a cat yet does not mean your chickens are safe. i don't think you should harm an intruding cat. But you do want to let it know it isn't welcome with a water hose or some other uncomfortable confrontation. If the cat continues to watch and stalk your chickens, trap it and take it to Animal Control. It's your job to protect your flock.