I've heard three theories as to why cats will kill little animals or bugs and leave them lying around for their people to find;
1.) The cat loves you and is leaving you a gift.
2.) The cat thinks you'll keep it safe for her, because she'll want it back later.
3.) The cat is basically trying to tell you "You're too god damn stupid to hunt for yourself so I did it for you. Idiot."
Which one do you think is right? If you have a cat that does this for you, why do you think they do it?