Foxes are not nice animals, they are one of the few wild animals who kill and mutilate their victims for fun.
When they break into a poultry coupe, they will kill everything in it perhaps carrying away only one carcase.

Myth. From The Fox website

Why do foxes kill for pleasure?

They don't. However, when the opportunity arises, foxes kill surplus prey even if they are not hungry and cache (bury) it for later use. This is a very sensible strategy in the wild, since there are likely to be some days when hunting is a lot less successful, and so the fox can eat the prey killed earlier. However, in an unnatural situation such as in a hen house, where the prey cannot escape, this behaviour, called "surplus killing", leads to the fox killing far more prey than it could ever consume.

Lots more very useful information and myth dispellation on that site too including

Overall, lamb losses to foxes are very low, and fox predation is much less important than other causes of lamb mortality. Higher than average levels of lamb mortality are associated with poor standards of husbandry, and so the most cost effective way to reduce lamb losses is better husbandry.

I suggest that everyone should avail themselves of this free resource from the Mammal Group of the University of Bristol.

DtG