How can people be so cruel?

I have to say, I'm a little flabbergasted by this alleged story of animal cruelty from Big Sandy, Texas. I know animal cruelty exists, but I always wonder how people can be so mean. Here are the first two paragraphs of the story, which appears in the Austin American-Statesman:

Two East Texas women face charges of animal cruelty after authorities found the remains of dozens of dogs in their trailer home, authorities said.
The dogs' bodies were stuffed into coolers, plastic bags and the freezer. Animal control officers also recovered nine living dogs.