They might have the ApocaFists equipped. Which do massive damage.hey nice mod! just found 1 thing. there are some civilians that can kill me with 1 punch. it doesn't happen often. i don't know if they have a melee weapon equipped but the damage is a bit too high i guess. i don't use any health point or damage reduce upgrades btw.
And there's no damage reduction upgrade for someone punching you in the face. You just gotta buffer it with a higher amount of health.