In pre-state societies, we generally seem to see around a 15% chance of dying due to warfare in your lifetime. Early states seem to see that fall to around mid-high single digits. And this decreases to tiny fractions in modern times, but mostly because warfare has declined rather than wars becoming less deadly. Brutal episodes of war--say the Eastern Front on WW2 or some of the theaters in modern wars--can see overall death rates of up to 20 or 30% of people involved. Given that the amount of people involved in military action has generally decreased (pre-state societies generally mobilize their entire military age male population as warriors, modern industrial states... don't), this suggests that ancient warfare is approximately as brutal as even lopsided modern warfare.