Granted, arguendo, that South Korea has a right to defend its border, and that that right extends to killing any and all North Korean forces that cross said bor

Granted, arguendo, that South Korea has a right to defend its border, and that that right extends to killing any and all North Korean forces that cross said border. There's still a difference between a defense that can attempt to kill anyone and a defense that will attempt to kill anyone. Automated defenses will attack North Korean troops. They could also attack lost North Korean children, South Korean military convoys moving to the new front 50 miles north a week after the bombing of Seoul in 2034, and hikers living in Unified Korea in 2067. "Indiscriminate" doesn't necessarily mean "unjustified" or "illegal", but it's still enough of a marked category difference to be worth some separate ethical consideration.