unless your adversary views your population as insects to be exterminated,
unless your adversary views your population as insects to be exterminated,
[unless your adversary views your population as insects to be exterminated,]
https://en.wikipedia.org/wiki/Battle_of_Thebes#Destruction_of_Thebes
https://en.wikipedia.org/wiki/Battle_of_the_Persian_Gate#Aftermath
Heck, even William of Normandy went after the local civilians when his retainers in newly-conquered England were ambushed and killed. His depredations resulted in a 75% reduction* in the population of the lands upon which he chose to inflict exemplary violence (to deter other challengers to his rule).
https://en.wikipedia.org/wiki/Harrying_of_the_North
The deliberate root and branch extermination of the entire population is another matter. That’s a fairly unusual approach to empire, and the likely cause of Hitler’s downfall. Whether Alexander or Genghis, successful conquerors tended to incorporate the armies of the conquered into their formations, to replace their losses, keep newly-acquired combat-age men busy and to balance out internal rivals/courtiers in their own high command. That Hitler chose to starve to death millions of able-bodied Russian POW’s he captured, whose fate at Stalin’s hands if repatriated to Russia was death (for surrendering instead of fighting to the end) or a long stint as a guest of Russia’s many gulags is evidence that he had serious hangups about ethnicity.
* Some of these people are said to have ended up in Crimea, of all places.
https://en.wikipedia.org/wiki/New_England_(medieval)