What do you mean surrendering prisoners? Do you even play this game? You took them by force in a battle, where you might have lost many men. Get a grip dude.
You mean literally like every war, where after one side starts to win, the other gives up and surrenders?
Germans on D-day surrendered after slaughtering Thousands on the beach, and while the Americans gunned down many of them after they surrendered, they still took prisoners. The entire point of the Nuremburg trials. Otherwise they would have just killed them all on the spot. Why bother with court. It's what "Civilized" society does, and has done for hundreds of years.
The promise of safety of your troops, so that they don't all desert you. If being Captured =/= death only the most Elite and zealous will stay and fight, the rest will run at the first sign of a loss. Awful for morale.
Another great example. Decimation -- If the Romans lost, they would enact Decimation where 9/10 would kill 1/10 of the group as punishment. The fear alone of being killed for losing, meant that there was no safety net. So they would fight more zealously, because even if they managed to escape, they could still be killed.
Russia during WW2, with commisars. "Not one step back, or I'll kill you myself". You may as well die fighting, because if you retreat you will be killed anyway.
Genociding entire armies after beating them was a really good way to get an awful reputation, with only the cruelest willing to stand beside you. The rest would either have to be forced via conscription, or straight up avoided you.