Because we live by two different laws. Jesus christ, regardless of how much you wanna say a cat is domesticated and there for we should interfere with it's nature to kill, it's still is an animal with brutal instincts. To suppress it, or interfere with it, is a wrong.
So you wouldnt interfere if you saw a wolf trying to eat a human baby?
Agriculture and Domestication both interfere with nature, so should we no longer grow our own food or domesticate animals?
Animals may have jungle instincts, but if they are living in urban areas, then people are free to stop them from doing anything they dont like, such as killing another animal.