I wouldn't kill a cat in my neighborhood as I know most (if not all) of them are pets. However, if I lived out in the country like I used to with no animal control, you have to do what you have to do.
She always fed them, when needed, as they rid the barns of rodents, insects (nothin' like a good, crunchy grasshopper), etc. .
The problem was in-breeding until she found a feral cat org that spayed/neutered them for free: here
Most places have volunteers that can help....before anyone shoots, check out the volunteers in your area.