why does roger throw stones with the intention of missing henry?

Roger is a sociopath in the making: he likes to hurt people. The fine line between the thin rules of civilization and reckless abandon of "nameless authority" quickly vanishes for this boy. He is content to throw stones at Henry barely missing him because of some unspoken conditioning in the back of his mind. This old line of society between threatening and hurting people is quickly vanishing in Roger's mind.