Serious question - where did this kid learn to handle a rifle? At 17 years old he did better than many that I knew in the Army. He couldn't have learned it playing Call of Duty; did he have a cousin or older brother who did a tour overseas?
I've looked at the different angles and read the interview comments and came to the same conclusion. Quick reaction in close quarters. Deadly. He certainly believed in supporting the law and order cause, so there's your motivation. If you have motivation, the skills can follow if you have mentors.