At the same time, this is an issue both terrible and important to think about.
B.H. Liddel Hart, in "Scipio Africanus," notes that the majority of causalities, fatalities, and damage don't occur during conflict, but afterwards -- to the losing side.
From Hannibal's defeat at The Battle of Zama -
"...the loss of the Carthaginians and their allies [was] twenty thousand slain and almost as many captured. On the other side, [the estimates range from "1500 Romans" to "2000 of the victors."] The discrepancy is explained by the word "Romans," for Livy's total clearly includes the allied troops. It is a common idea among historians that these figures are an underestimate, and that in ancient battles the tallies given always minimise the losses of the victor. Ardant du Picq, a profound and experienced thinker, has shown the fallacy of these cloistered historians. Even in battle to-day the defeated side suffers its heaviest loss after the issue is decided, in what is practically the massacre of unresisting or disorganised men. How much more must this disproportion have occured when bullets, still less machine-guns, did not exist to take their initial toll of the victors. So long as formations remained unbroken the loss of life was relatively small, but when they were isolated or dissolved the massacre began."
Could there more mercy, more clemency? Well, we would hope so, but then there's the faction/negotiation/diplomacy problem... when the defeated surrender, they only represent one faction who is at possibly the lowest point, most conducive to surrender. Later on, once cleaned-up and re-armed, the war-hawking faction will return, and if the opponent's forces are not reduced in defeat, they will likely be re-deployed and battle will began anew.
I'm reading The Grand Strategy of the Byzantine Empire. I wouldn't recommend it if you're new to history, because it assumes a very high baseline of geographical and basic historical knowledge, and it can be dry at times. But if you've read some books on Rome before, and especially the Byzantine era of Eastern Rome, then the book is packed with gems on strategy, analyses of organization and logistics, culture, interesting stories and anecdotes, and the personalities behind the vast and epic clashes of the era.
This passage about Attila is insightful --
It's commonly recognized that one of the reasons that Hannibal lost to Rome was that he was unable to get full support from the Carthaginian aristocracy to fully back him with men, materials, and money.