I didn't know why it would be it just feelz wrong. So I googled some things. I guess this kinda fits my feelings of why it would be:
"The action must be necessary to achieve a legitimate military objective. If the retreating soldiers pose no immediate threat or if their destruction does not contribute significantly to the military objective, the use of force might be deemed unnecessary."
But ultimately I think it boils down to war being heinous and I don't like people dying especially by a voiceless, human less, robot RC device. Just feels dystopian.