I serverly doubt this guy is ex-military, for a couple of reasons:
1. the kid was just injured, military snipers dont miss
2. The kids mom her the blast, a military sniper is not goin to randomly shoot some kid close enough where somebody might have heard or seen where the shot came from.

This guy is just some nut job, who probably thinks he has some greater cause, or maybe he is just mad at the world. I just hope this doesnt progress like the School shootings did, random violence all the way to organized torture and massacre.