If we are to believe some of the old documents, slaughter like this may have been relatively common in the days of no quarter. Even as late as the 12th Century, Edward Longshanks killed every man woman and child in the city of Berwick to put down a rebellion by the Scots.
Roman historians regularly give accounts of battles where whole armies were destroyed by Julius Caesar or Pompey for example.
And the mongols who conquered Baghdad reputedly killed 750,000 people while wiping out the city.
The Romans are said to have suffered 50,000 dead at Cannae. They claimed to have killed 80,000 Celts near Watling Road when they ended [for all extent and purposes] Bouddica's revolt. They lost in excess of 15,000 [probably closer to 20,000] at Teutoberg Wald. And their losses at Carrhae, at the hands of the Parthians, were much higher than that.