icy sundial
It looks like you only skimmed by his head with 2-3 shots total, idk if you're aware, but the bullets always go to the center of the screen in The Finals atm, the sight of the gun has nothing to do with where the bullets are going.
To answer your question posed, however, always aim for the head so long as you feel comfortable, the more headshots you are hitting, the more likely you are to come out on top in an engagement. If you don't feel like you are going to hit the head enough times, aim for the body for consistent damage, but always be aiming around neck-level so that you can easily transition between the 2. You will learn how many times is "enough" times as you get used to the damage per shot and timing within each kill in the game and as you get used to the recoil of each gun you are using as well.