I read something that might explain why mankind enjoys slaughtering (not only at bullfights, but there are many, many examples, including bloodlusty berserkers).Said there our ancient pre-human ancestors were descended from grain-eaters. They didn't have hunting-instincts like predators. When they reacted to the changing world by starting to eat everything, including animal meat, they had to devellop new hunting skills based on self-defence and social rank fights. Predators don't show agressions while hunting, but keep "cool". On the other hand, for example wild stallions fighting for social ranks are not only very agressive but tend to serious injure each other.
Now human race combined the "killing instinct" with "rage" instinct.
This would be the reason why humans feel joy or tend to get furious when wounding or killing other animals, including other humans of course.
Of course this is no excuse. We have the gift of intelligence and can thing about good and bad. This is where human kind really has the chance to raise above the animal.