Well, here's JMHO-
We are not part of the 'natural' ecosystem either. We build houses with shiny windows that songbirds fly into and break their necks, we spread chemicals on our lawns that kill insects (then feel good about ourselves when we fill our birdfeeders) and wash into our rivers that in turn kill fish, turtles and amphibians, we drive big trucks on habitat-dividing paved roads that slaughter raccoons, opossums and what-have-you, we introduce non-native species and then trap and kill the native species that are just hungry because we've plowed down their habitat to build a
Walmart where you can buy rat poison, Scotts Weed and Feed, raccoon traps, hunting ammo, beer and birdseed all in one convenient stop.
Most people care nothing about the natural ecosystem except when it serves their purpose. We can justify killing feral cats with lofty arguments such as they impact native wildlife and spread disease but lets face it, the
real reason is because we're looking out for what is ours. Humans rarely do anything unless it is of benefit in some way to themselves. This is the unflattering truth about ourselves.
I'm not saying feral cats are not a problem - I'm in animal rescue and I know the frustration. If you eradicate a population then another population moves into the void since food sources have become available. It's crazy. Take the cat to a rescue group and at least give it a chance. A shelter will most likely euthanize it, they rarely have the time or the resources to deal with ferals.
I agree with NoelTate, we've had great success with this and feed several stable colonies currently but sometimes people just don't believe in having cats around. Some people just flat-out don't like them. Those voids will always be death zones for these animals.
It's not the cat's fault, it's just trying to survive a bout of someone's irresponsibility, ignorance, cruelty or neglect.