[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.]
read the study yourself. The most commonly preyed-upon species by cats is birds. It's a global problem. I will grant you that perhaps cats fit slightly better into the urban ecosystems of Europe, but this is a minor point. They still have an outsized population due to human influence, so they will always be massively disruptive to local ecology. And none of this is considering the danger posed to the cats themselves, whom we all love.