What does Stanley say to Zero in chapter 18 that is a sign of a hard heart?

Over the course of his long stay at Camp Green Lake, Stanley has toughened up in many different ways. As well as developing physical toughness, he's also developed something of a hard heart. You'd think that someone like Stanley, who's been mercilessly bullied by so many different people over the years, would display a little more kindness to other people. But by the time we've reached chapter 18 of Holes, Stanley's whole character appears to have changed, and not in a good way.
One night, Stanley is writing a letter home to his mom. Zero comes into the tent and eventually asks Stanley to help him learn how to read and write. But Stanley is not interested; he coldly tells Zero that he's sorry, but he doesn't know how to teach. Zero is not really asking for anything much, but Stanley won't even try to help him:

After digging all day, he didn't have the strength to try to teach Zero to read and write. He needed to save his energy for the people who counted.

And that's just it: Zero doesn't really count for Stanley anymore. Life at camp has hardened his heart as well as his muscles and hands.