Really? "wild animal"
A dog is just like any other animal on earth. If you provoke it it will attack. Most "pitbull" breeds have a better tolerance so I would assume this was a constant teasing. And there is no way I would put my dog down for his behavior, I would ask him if he learned his lesson though.
This is no different than him picking on a kid at school and that kid beats the crap out of him and maybe splits his lip so he has to get stiches or maybe breaks his nose. Would you kill your kid for that? I would assume not. My guess is this kids parents don't teach him respect for anything.