I have to agree with everyone else, it was simply a miss. At the time you fired he was crawling, your crosshairs were over his leg and you missed.
As far as sniping on this game goes, I see very few problems with it. A sniper rifle isn't a run and gun weapon, and quick/no scoping is not sniping. Trick shots are cool and all, but they aren't going to win many games. I know there are videos all over showing trick shot kills, 80% of those are setups or edited a few are displaying actual skill and the rest are luck.
Use the rifle as it was intended, stationary from a distance, and the one shot kills will roll in.