Now this I agree with - the level of road kill is truly shocking and really sad to see.
Quite a separate issue to what's being discussed here though, and cats clearly do kill songbirds in large numbers. I say put a bell on your cat if you care about birds in your garden.
What a ridiculous point to make!
Manure is from herbivores - less harmful germs in manure than in poorly cooked meat!...
..which leads me onto toxoplasmosis. The chief carrier of which are cats, mainly because they are carnivores (not exclusively because of this though).
In short, manure...
You also don't have to report it if you run a cat over with your car, whereas with a dog you do. Not that I'm advocating running any animal over of course, just pointing out the law.