Yes, they do have a few predators, things like the goshawk and buzzard. Further afield, other owls like the ural and eagle owls. But a predator would probably have left obvious signs, and more likely it wouldn't have left the prey for you to see unless you happened to have disturbed it. A lot of birds are deliberately (and of course illegally) poisoned, and some may die from eating poisoned vermin although I don't know how big an issue that is. Many die because they ingest man-made things like plastic rubbish which their insides can't cope with. And of course there are natural causes. How big was it? Any photos?