There are a couple of theories on why cats play with their prey, and neither of them are "Felines are sadistic monsters"
1) They want to wear out their prey so that when they go in for the classic feline kill move of chomping on the prey's throat, they don't suffer a counter-attack. This seems plausible because even though the typical prey of a house cat are small, they're not incapable of delivering a nasty bite or peck when their life is on the line. A wounded predator can be in a lot of trouble and so it makes sense that they might develop behaviors to avoid the possibility.
I had a big black Tom that brought us rat carcasses almost as big as him. I doubt he played with the rats before he killed them as the battle had to be epic and rats are not wimps, but I never, alas, witnessed one of his kills.
2) They are taking the opportunity to practice their killing skills. This only makes sense to me from the basis that they are typically well fed and are killing out of instinct, not hunger.
Of course we can't read a cat's mind so I suppose there's always the possibility that they are cruel sadists. I don't think anyone, who knows anything about animals, seriously believes this might be true, but if it can't be totally ruled out, I guess it's open to use by the uninformed.