They didn’t need advanced weaponry to kill tens of thousands of people. A few thousand soldiers would go through a city, house-by-house, and put every single man, woman, and child to the sword. They then looted what they wanted and burned the house to the ground.
Thousands more died during the siege, long before any soldiers reached them, due to disease and starvation from overcrowding and dwindling food supplies. It was absolutely horrific and it happened mainly because swords, spears, and bows were useless against city walls so long sieges were the rule.
The massacres that followed could be chalked up to deep resentment on the part of the attacking soldiers for the defenders holding out so long. I think another part of it is simply that the commanders of the day had less power over their soldiers, so looting and pillaging was part of the bargain to encourage them to fight.