"Fellow students say he had anger issues."

Strange the way that kind of language has crept into the way people describe what happens.

Words... I imagine if some murderer were to take an axe and chop a bunch of people up, and it was described as his "butchering them", there'd be people protesting that he wasn't a qualified member the meat processing industry, and that to call him a butcher was to malign a fine body of citizens.

A sniper is someone who kills people with single shots fired from cover and from a distance, without giving them any warning or any chance to hide or respond. But having a professional qualification or being employed by the army isn't part of the definition. It's just a matter of having the skills, and the motivation. And of course a particular type of personality - a sniper has to be able to be totally detached and unemotional in the killing, or the bullets would miss "the target".