By "explain" I hope no one hears "justify" (although someone will do it anyway):

You see this with the Romans, too: they tell a city to surrender, it doesn't, the Romans conquer it and practice unspeakable brutality on whomever's left.

So what's happening? They have very limited resources, you forced us to expend them, maybe lose some of our own soldiers, and waste a valuable campaigning season - ok, now you pay the price, and let this be a warning to the next city: if we tell you to surrender, you'd better surrender.

What if the city does surrender? Are those people killed or sold into slavery anyway? I don't think historians bother recording those episodes.

"What if the city does surrender? Are those people killed or sold into slavery anyway?"

Typically, no. The attacker may leave the city entirely intact, or may place their own proxy government. They may take the elites of the city as prisoners, either as punishment or as insurance.

It's not quite so simple, since we are looking at single examples. Alexander was predated by leaders (Cyrus the Great) who ruled in a much more progressive gentle way. Recent history has shown major powers performing very cruelly (genocides under colonialism) and there's definitely at least minor world leaders who would want to be just as cruel. So the question becomes more of why was it effective and strategic to be cruel at that scale in the past but not in the present world, and I would attribute that to technology: better communication, travel, and interconnectedness of trade.

Cyrus the Great gets a lot of great press. Partly because he replaced really unpopular empires. And party because those that he really didn't like didn't give us many sources.

Its very questionable if he was all that nice to the Medians and Assyrian cities.

I would attribute it to democracy. It's very hard to sell war to an electorate unless you can persuade them you're the 'good guys'. Which is harder to sell when you're openly butchering thousands of people.

Many ancient societies had a very different conception of 'good guys' than the modern one. It was very easy to justify very aggressive war with some of these alternative conceptions.

'Good guys are the ones who worship the right god(s) and therefore we get to kill the bad people next door', or 'Good guys are guys who serve the King/Emperor and anything that advances his glory is good', 'Good guys are cultured people and everyone else is a barbarian' etc.

Democracy didn’t stop the Athenians from exterminating the Melians, every adult male being killed and the rest of the population sold into slavery, all because (according to Thucydides), “the strong do what they can and the weak suffer what they must.”

Athenians weren't really democratic by modern standards. Only about 10% the population could vote[1].

[1] https://en.wikipedia.org/wiki/Athenian_democracy

But why and how is democracy a thing?