I was watching THE WITCHER on Netflix, and when the Nilfgaardian empire conquers Cintra, we see them burn everything to the ground, kill every civilian they encounter, and we're told that this is basically their M.O. whenever they conquer someone. And that...seemed kind of bonkers to me?
It was my understanding that slaughtering civilians in a feudal society would quickly reach a point of diminishing returns, since a) you need them to work the land, b) agricultural output is your primary source of wealth, and c) heavy cavalry is your primary source of military strength and horses need to eat a lot.
[Granted, as with most "medieval" fantasy, The Witcher is kind of a mishmash of late feudalism and early renaissance, and it's also not immediately clear in the show whether Cintra is a kingdom or a city-state or somewhere in between. But that that being said...]
Edit: some typos fixed
Such a "scorched earth" approach is vividly portrayed by Orderic Vitalis in his 12th-century Ecclesiastical history (here in vol 2 of Forster's translation, Bohn, London 1854) in his description of William I's "harrying of the north" in 1069-70 following an English rebellion in support of the former Saxon royal house:
Never did William commit so much cruelty; to his lasting disgrace, he yielded to his worst impulse, and set no bounds to his fury, condemning the innocent and the guilty to a common fate. In the fulness of his wrath he ordered the corn and cattle, with the implements of husbandry and every sort of provisions, to be collected in heaps and set on fire till the whole was consumed, and thus destroyed at once all that could serve for the support of life in the whole country lying beyond the Humber. There followed, consequently, so great a scarcity in England in the ensuing years, and severe famine involved the innocent and unarmed population in so much misery, that, in a Christian nation, more than a hundred thousand souls, of both sexes and all ages, perished of want. On many occasions, in the course of the present history, I have been free to extol William according to his merits, but I dare not commend him for an act which levelled both the bad and the good together in one common ruin, by the infliction of a consuming famine. For when I see that innocent children, youths in the prime of their age, and grey headed old men, perished from hunger, I am more disposed to pity the sorrows and sufferings of the wretched people, than to undertake the hopeless task of screening one who was guilty of such wholesale massacre by lying flatteries.
Medieval chroniclers certainly weren't above wild exaggeration, and I long assumed that this was another case of the same, but the reported toll seems broadly consistent with the low populations recorded in the affected counties by Domesday sixteen years later (see Darby, Domesday England, Cambridge 1977). And as you predict, the Inquest duly indicates a severe loss of agricultural output and subsequent royal income - perhaps the reason why William's successors never repeated the exercise in England, at least on such a scale (though areas of Ireland suffered such destruction as late as the 17th century).
Has much changed? Except for regimes bent on ethnic destruction, successful armies still seem in the century of "total war" to have avoided mass extermination of whole populations or destruction of material resources of potential use to their own side, "scorched earth" being generally a tactic of the retreating rather than the advancing side.