Kids that behave like this often has growing mental issues. There is this saying in the medical world: "Kids that start torturing animals at an young age, sometimes go on to do the same to humans.."
Hope it was an one time occurence
It was very good of you to yell at the kid and make him stop the cruelty, if we all were to turn the back at such behaviour- then who are we really?? A bunch of chickens? Its okay as long as it doesnt happen to my dog, in my yard??
I would NEVER turn my back to someting like this, i would explode in the fraction of a second.