The killing of feral cats is so much more than a simple legal question and answer. It may be legal when protecting your property, such as feral cats engaged in the taking of your livestock or even pets. Taking a feral cat in the wilds of Idaho would require a valid hunting license.
However, there are other considerations to keep in mind:
Are you in a city limit that prohibits the discharge of a firearm? Does the county have a no shooting safety ordinance for your neighborhood?
Is the cat actually Feral, or has Muffy escaped their yard and the 8 year old owner out looking for their pet kitty?
It is impossible to provide a definitive answer to this question without knowing the specifics to your circumstance. And Fish and Game may not be the source for that definitive or even the specific instance knowing that each city and each county may have other provisions and therefore other solutions for removing the pesky cat in your yard. Other solutions could also include contacting your local Animal Shelter for assistnace.