Considering the number of mice my cat has killed, I tend to disagree that they have no concept of death at all. The extent to which they "understand" it is open to debate for sure. But whenever my cat catches one, he deliberately leaves it alive for a while to play with until he gets bored, at which point he promptly cracks its skull between his teeth to finish it off and tries to eat it(I usually stop it if I can so he doesn't get parasites). There's certainly some concept there. Probably not the same concept we have, but it's there in some form.
One theory as to why cats toy with their prey is they are tiring out and confusing it so they (the cats) reduce their chances of getting injured when they close for the kill.
That's an interesting theory, though it doesn't match my experience. My cat tends to bring the mouse inside through the flap, holding it in his mouth, and then will repeatedly grab it in his mouth, move somewhere else and let it out again. He has the option of killing it just by biting down at any moment.
My own hypothesis is that cats catch prey and play with them for a while as a way of getting more training per kill. They probably find some enjoyment in it, which is likely evolved as an incentive to practice. Cats are certainly well known for their playfulness, which tends to be stronger at a young age.
I suppose both theories might be true at different times, depending on the context or the prey. I guess mice aren't particularly capable of doing any real harm to a cat.
