Cosmoline,
Have you ever read "The Art of War" by Sun Tzu? If not, I recommend it. It will tell you why not.
There are benefits both tactical and strategic to avoiding extremes.
Wounding folks takes up more of the enemies' resources than killing them.
Even when it's a brutal dictator's soldiers, they are human beings. The brutal dictator will be forced to provide some level of medical care for his wounded. If he does not, it will destroy the morale of his troops. In either scenario, we benefit more from wounding than from simply killing.
There's an advantage to leaving your opponent a way out of the trap...or at least what appears to be a way out. You really don't want to face a trapped opponent with no hope of escape or surrender. Ever faced a cornered rat.
I've been beatened badly four times in my lifetime. Twice I was in a fight and forgot to check my six. Once I was assaulted with no warning from behind. The other time, I made the mistake of forcing a fight for the one time of my life. I chased him for about six blocks. He tried to cut through a fenced tennis court. I cut him off before he could reach the exit and hemmed him in a corner. After running for six blocks, this terrified pipsqueak turned around and tore me a new one. I'll not make that mistake again. Hell, I won't force a fight again.
Many, if not most, of the provisions of the International Law of Ground Warfare are to our benefit.
Oh, yeah...archaic? The conventions are not that old. You might want to check out the practices employed before then.
Romans? If a city did not surrender before the first battering ram touched the wall...the citizens of that city had no rights. It was open to sack: the men killed or sold into slavery. The women and girls raped and sold into slavery. The children sold into slavery if old enough...if a child was too young for an economical price he or she was killed on the spot.
Is that an improvement? Oh, and that was pretty much the way it went in Europe, except for the slavery during a sack of a city up until about 1700 or so. Instead of selling the survivors into slavery, they were usually just killed.