They're used in the bullfights which climax in the killing of the bulls, by the matadors, by repeatedly driving long swords between their shoulder blades down into their chest cavity until the heart is pierced and they die, the lungs are pierced and they down in their own blood or they internally bleed to death. The bulls are too muscular to eat, and are pumped with muscle enhancing drugs as they grow so the bodies are just thrown away.
Originally, in bull running, the bulls were transported during the ceremony to a bullring where during the evening, they would be killed. However after the run, the animals are usually led to a bullfight for entertainment in which animal activist greatly protest. Read more details at en.wikipedia.org/wiki/Running_of_the_Bulls.