To add another anecdote, my family once raised quail and we had a cat that would spend a good deal of its time sitting by the quail cages. One day someone left a cage containing 20 quail unlocked. When we came back, we found that the cat had slaughtered all the quail, except for a couple that apparently managed to escape the carnage. Clearly, the cat did not eat all of them.
There have been a couple of studies that have demonstrated this sort of behavior in cats and attempted to explain it, although I can't lay my hands on them right now as they were done in the sixties (fifties?). In these studies, cats appeared to have an innnate hunting instinct that, if prevented from hunting for a long period of time, caused them, for want of a better word, to go crazy, whereupon when they were allowed to hunt, they killed far more than necessary or typical for them. It is worth noting that the cats that did this were ones that had been raised as hunters. Cats that had never hunted live prey growing up did not show this phenomenon.
I also had a couple of cats that would find crickets, tear their back legs off, and then bat them around until they died. These were adult, well-fed cats, and I think it significant that they lost interest in the crickets once the cricket stopped moving. From this and the dolphin/porpoise videos I have seen, I can only conclude that they are indeed just having fun. It can certainly be attributed to instinctual behavior that is training them to hunt or some other functional behavior just as Alistair says. However, it is also possible to attribute any action taken by humans in a similar manner. I conclude from this that either one accepts that human feelings are only instinctual responses, or that animals do sometimes enjoy assaulting and sometimes killing other animals just like humans do. It does not make sense to me to say that humans are unique in this respect.