Imagine if his motivation was to prove he had great aim and that they did him wrong. Maybe he just wanted to become the most well known sniper-assassin in the modern world. Maybe that's the reason he didn't use scope, he wanted his memory to be that of the rifle maverick. I wonder what went through his head after he realised he missed. I mean, other than a bullet.
He looks like he commented to someone doing this sport professionally that it look easy and now, with a bit of disdain and irritation, he had to show them what he meant by that