I think there's a bit of confusion regarding their predatory instincts. It may seem they kill for the sake of it, but really it's a wild animal that has no idea where it's next meal is coming from. Foxes are fantastic opportunistic hunters - which is just that, opportunistic.
If we had to hunter gather our food, a fair few of us would take more than we need, being apprehensive of leaner times (eyes bigger than the belly scenario). It's the psychological reason why most animals overeat, or overkill, when they do. Rabbits will do it (with pellets) and become fat even with exercise, apes are a classic example, even reptiles and fish do it. Even the beloved domestic cat does it (horrendously excessively too) even though, being domesticated, they do know where their next meal is coming from in the vast majority of cases. Cat's are a much worse example of killing when they needant, and leaving the carcasses - and they have been domesticated for a long long time.
The reason cats still do it is the strong instincts that even domestication hasn't be able to get rid of. The very same instinct (but stronger) in a fox. They are driven to act upon potential prey to survive. Interestingly, the only mammal so far to have been proven to kill for enjoyment or for the sake of it, is the Bonobo Chimpanzee.
Totally agree with Shimmer, don't underestimate them, they are predators and very skilled ones at that. However also very much agree with vegan bunny. They're not killing simply because they can, they're killing for a reason - even if that reason isn't always clear to us when they leave prey. They're not evil animals, they're just trying to survive and don't know when they can next get food. X