what should I do?

We don't have foxes here so I'm not familiar with their behaviour but maybe use what's left to bait a live trap? I know it sounds awful but at least she wouldn't be wasted and may save her flock members. If you don't want to kill it or it would be unsafe to do so (shooting in a residential area), can you call animal control or a similar group?

 

Be sure you live in an area where it is legal to discharge a firearm before shooting the fox. You can try baiting a live trap or leg trap with the dead biddy. I use a live trap for predators. Then I put the entire trap in a garbage can and head to the grocery store and buy a couple of blocks of dry ice and put them in a cheap sytrofoam cooler that fits inside the garbage can, over the trap.

I put a piece of cardboard over the trap, just large enough to keep the dry ice from burning the predator and small enough to allow the CO2 vapor from the dry ice to drift down to the predator. I then tip the dry ice into a steel bowl on top of the cardboard, add a cup of hot water, and put the garbage can lid on as tightly as possible. I come back in an hour and take a quick look to make sure that the dry ice is still evaporating, if not, I add more hot water. The CO2 asphyxiates the predator, which you may then process or dispose of as you see fit.

Some people use the CO2 dispensers used for beer kegs to achieve the same effect.

I don't use leg traps because I live in California where you seem to need a trappers license to set anything other than a mouse or rat trap. It is illegal to even use dead traps if you aren't a trapper in California.

 

In the long run, its easier and more effective to put an electric fence a ways out from your coop and run. The fox is just the predator that you saw, the night is alive with predators looking for dinner, and by putting an electric fence around and out from your coop and run, you keep them away, all the time, not just when you see them. Also by putting it a ways from the actual coop they don't get close enough to be super orientated to the fact that there are lunchables inside. Electric fence is cheap and easy. The most important thing is to ensure a good ground.