I guess it was standard military practice to slaughter entire cities, not just the Romans.
I think the Romans usually tried to avoid that kind of waste. Julius Caesar sold thousands of Gauls into slavery after the battle of Alesia and became extremely wealthy because of it.
There was often a lot of bloodshed, back then, they weren’t all gumdrops trees and sugarplums as conquerors are today. :’) However, most people inside a siege were not fighters, they’re just home having a cookout when the whole thing started, and they didn’t usually have a summer place to go to and lay low for a while. IOW, people would surrender when the walls were breached.
It wasn’t unheard of for fortified cities to just bag it and invite the conqueror in, y’know, if it appeared that the invading army was enormous and had siege equipment, or the crops were still in the field (that’s a good time to go be a bloodthirsty conqueror), or the menfolk were away (military operation, or more likely just out doing something related to food, herding, or trading).
The Assyrians would just march right in, and after the city either surrendered or was taken by force, they’d levy an annual tribute. The tribute would be used in part to pay off the financing for their own conquest, which is kinda rude, really. The longer a town resisted, the more expensive it was for the Assyrians, so the tribute would be higher. That made it more likely that the town wouldn’t be able to come up with the money, and they’d just say, screw ‘em. So the Assyrians would have to go back.
The Assyrian army was unlike most great empires in that it retained what Jimmy Carter would call its ethnic purity. They persisted for a long, long while, marching all over the Near East, into N Africa, Anatolia, and perhaps as far north as the Crimea. Some of the later Assyrian kings didn’t spend a lot of time besieging a city, and just brought overwhelming force to break the place in a matter of days. That saved a lot of marching later.
Of course, there weren’t that many people sad to see them go when the Babylonians, Scythians, and Medes joined forces and stormed and sacked Nineveh.