The easiest way to avoid cats killing other animals? Keeping them indoors. The kitty gets to live, they live a safer life, and they don’t kill other animals. You cannot feed a cat a vegan diet, but you can avoid some death on their part by letting them live a life indoors. No animal is more important than the next, so that way, everyone wins. The sad side-effect of that advice is the many cats that won’t get any mental stimulation or possibilities for exercise despite the good intentions of their owners. Depression and poor physical form is a very real issue for many cats ( and other pets btw)
I worked with a girl several years ago who had two cats and was very zealous vegan. She did not drive cars or ride public transit only bikes. Very environmentally conscious. When she brought up the cats I asked her how she felt about feeding them and what they ate because I had never had the info for the conversation before. She said she fed them avocado and a mix of other blended vegetables. they both became very ill and were taken from her for animal neglect. Or, she may just be a young, idealistic vegan who is not very realistic. That cats eat rodents may be sad but it is a great service. if other animals didn’t eat rodents, they would breed and breed, and we would all get sick and die.