[alt text: an illustration of a person with a head-empty expression on their face, who is saying, "Not letting your cat outside is CRUEL!" Around the person are various gruesome scenes of different cats in distress. From the top and going clockwise, the scenes include: a cat being carried away by a hawk; a cat that is on fire; a dead cat in the road that has been run over by a car; several dead kittens; a cat that is missing an eye and various patches of fur; a cat that is feasting on a songbird; and a cat that is being carried away by a coyote. The person appears to be completely oblivious to these scenes of distress.]
That depends entirely on where you live. Cats are pretty much native animals here in Europe.
Tell me you’ve never owned a cat without telling me you’ve never owned a cat.
even if they are native animals, the SIZE of their population is caused by humans, and so they are disruptive to their local ecology. and bell or no bell, letting your cat outside unsupervised is leaving them to get hit by a car, or tortured by mean kids, or scooped up by a predator, or stealth-adopted by someone who cares about the well-being of cats more than you do. I'm assuming you wouldn't be happy with any of those things happening to your cat.
I own two cats, and they both have bells. They haven't killed any wildlife since then. Cats are not native animals in Europe, they are very much invasive and devastated local populations of birds and small mammals.