Cat's bring you dead animals for two reasons:
1. Many domestic don't actually realise they're supposed to eat them. Seriously, cat's have to learn from their mothers to eat mice, birds, etc. Catching them is an innate instinct; eating 'em isn't.
2. Cats never see you hunt; so they bring you in dead, then injured then live animals to practice with. Seriously, if you observe the behaviour of a mother cat with her kittens and a cat with her owner they're identical, down to body language and everything. This is why female cats are more likely to bring in whole animals, and male cats are more likely to leave severed heads and the like (generally somewhere you'll tred on 'em when you come down in the morning without your shoes on; don't know why they do that).
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