This is standard, though. Cities don't simply surrender after a siege begins. The idea anyone was even entertaining to accept a post-siege surrender, and not the massacre, is the unusual part. The only way it would have been more or less historically feasible is if the Lannister army stormed the Red Keep and tossed Cersei and Euron's heads in Daenerys lap after the siege began, but before the walls were taken.
Daenerys was always more enlightened than 99.99% of medieval rulers, who most certainly would have destroyed and killed far, far more than she has. Of course, that's before obliterating King's Landing as if she had been Genghis Khan or Cato the Elder all along.
You could try to argue they have developed different morals, but everything we know about Westeros and Essos such as the destruction of the Castameres and everything sadistically done to slaves in Essons point to these people being even more depraved and vicious than their real life counterpart cultures.
Cities may not simply surrender. But this one have after the destruction of only some strategic portions. And the army just dropped sword. After that Dany goes berserk (I don't mind as I was expecting it, but as said the trigger was a little odd) and her soldiers start killing unarmed surrendered guys.
Yes Dany is less savage than those rulers, killing the slave owners and opposing families aren't that problematic, but it were cracks in the armor that she wasn't perfect and lovely ruler.
duduspace11 "Well, since we are estimating costs, Pokemon Red/Blue did cost Nintendo about $50m to make back in 1996"
Mr Puggsly: "Hehe, I said good profit. You said big profit. Frankly, not losing money is what I meant by good. Don't get hung up on semantics"