Saying that "Humans are cruel" isn't a truly accurate statement. It implies a differentiation between humans and other organisms. It's not that humans are crueler than other organisms, but that nature herself is much crueler than what many people would anticipate. For that matter, generalizing humanity is kind of a hard thing to do. If you've met one ant, you've met them all. However, the human brain facilitates minds as wildly different as Adolf Hitler and Buddha.
And back to other organisms, many animals regularly commit acts of cruelty that would be unthinkable to most human cultures. For example, Chimpanzees eat infants in their own species. Also, try hugging a panda bear.

Anyways, on the topic of human cruelty to animals, it's obviously wrong to burn kittens for our sadistic amusement or even pull the legs off of a spider for fun. Now, I for one believe that animals exist for the sole function to provide for humanity. As such, your average dairy is not a pointlessly cruel endeavor, and ultimately the right of humanity. However, something like forcing pit bulls to fight each other to the death and then electrocuting the loser is no longer utilization of resources, but throwing fuel into the fire of sadistic urges.