Do cats eat mice that they didn't kill themselves?
When they are kittens, definitely. Their mother kills mice and
rips the mice to pieces to show the kittens what to do. Later she
brings injured live mice to the kittens to teach them to hunt.
When they are adults, it will depend on the cat.
House cats often will not even eat mice they killed themselves
as they were not trained by their mother to do so.