Farmers will kill them because foxes will kill their chickens. Usually a fox will only take one chicken and run away, unlike other animals such as weasels and skunks. They will kill them all and then start to feast. Now which is more sly. The natural threat to foxes are eagles and coyotes.
Foxes have an incredible abilities to protect themselves though.
They have stupendous hearing. A fox can hear a watch ticking thirty feet away. A fox's ears can turn sideways to hear better . Actually a fox mostly hunts by sound not by sight. They also have an acute sense of smell.