I just did the same thing yesterday for a neighbor up the road. A big old tom started hanging around and eating their dog's food. They started putting food out for it up on a side hill about 60 yds from their house. I got the call yesterday morning to come dispatch the animal. Used my 223. That cat was huge, stank to high heaven, and had a ton of scars.
I don't hesitate at all to kill feral cats. They do a tremendous amount of damage to our grouse, woodcock, and songbird populations.
SG