Why do people kill cattle?

It's all because of an ancient predator-prey relationship that is always at play, even at the domesticated level. Humans are predatory animals, cows are prey animals. Cows are killed for their meat because humans, being natural predators (though not in the same sense as wolves, wild and domestic cats, Spiders, and many other predator/carnivorous species are), like to eat meat from cows, which we call beef.