There HAVE been studies.
A fox, like most predators, will get pretty excited when in close confines with a lot of pet species, and will get into a bit of a frenzy. Especially young animals.
But it's an in built survival mechanism, as we like to say "make hay while the sun shines", something kicks in, and they kill as much as they can while it's on offer, and as you say, bury it for later.
They are canny creatures though and for the most part, they don't feel safe carrying them all off, and give up after a couple, hence they leave so many.
Some animals do kill for 'fun', well maybe not fun, but for the joy of killing, mustelids are a good example, a weasel or stoat will decimate a pen of poultry without the hope of carrying even one bird off.
But then again, what creature that you know, kills only what it needs if given a chance? How many cats would only kill the one mouse or bird it needed?
I do like foxes, they are one of my favourite animals, but I also see the damage that they can do if uncontrolled. I was out last night on a call out, a free range chicken farm with automatic pop hole closers. . . . had a power cut on friday night and the pop holes opened at night! You can imagine the mess the fox made. So I sent a few hours out in the rain last night, had the dog fox, but couldn't take the vixen as she wasn't in a safe position. So it's back out today or tomorrow sometimes (as soon as it starts to rain heavy) checking earths in the day.
Killing anything can be an un pleasant task at times, but it's part and parcel of the countryside.