The Mongols were generally fine if you submitted. If you put up resistance or kill their messengers then they would wipe the floor with you and everything you owned.
That pretty much describes ancient warfare as a whole. First, they gave you a chance for total surrender. Then they would give you a chance to send out the non-combatants before they attacked your city (the Mongols may have skipped this step, but the Crusaders were sticklers about it). If you refused to do that, then everyone in the city was considered a hostile and were fair game in the ensuing battle. No one was spared because it was assumed they if they had refused to surrender, they would also kill you if they could. There was no way to create POW or refugee camps so they just killed you, or if they had the opportunity made you a slave.