Foxes will only kill chickens if they can and it's your duty to ensure your chickens are safe. Foxes don't carry keys or wire cutters, if they can't get in, they can't cause any problems. A fox, given the opportunity will kill more than it will eat in one sitting, this is not killing for pleasure, it's common sense. Kill when it's available, no different to us going shopping, rarely do we shop for the day, we shop for the week and store the rest!
I personally think it is cruel to kill a fox who could have cubs which would starve to death, but thats me, I wouldnt kill anything. Keep the chickens in a secure area, both day and night, and you shouldnt have a problem. It is illegal to catch a fox and relocate it to another area due to mange, and it wouldnt survive anyway, as the other foxes are territorial and would kill it.