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.
This is a terrible thing to consider, perhaps one of those hard parts of human nature, and one of the most difficult aspects of command. You must consolidate your victories and finally remove the opposition's strength and material, but seems such a damn waste that all the carnage has to happen after the issue is decided. And yet, the faction/negotiation/diplomacy problem persists, which causes this, and it's human nature at its core.
Terrible stuff. I suppose "Don't lose" is too obvious of a lesson to write, so we'll refrain from saying that.