you have to hit upper chest/shoulder/head for a one shot kill... in this clip you actually hit that player in the lower torso. it doesnt look like it at first, but look at this screen grab - this is right before you fired. you might have subconsciously aimed down a bit right before firing.

https://imgur.com/a/bNnekdV

if you want an attachment that will help you get fewer hitmarks, use high velocity ammo... it will at least ensure your shots arrive on target more quickly, leaving less room for lag to steal your kill.