What's a "battlefield"? World War 2 was fought all across the cities and suburbs of Europe. It's been centuries since people flew banners and lined up to whack each other in the heads with halberds in an orderly fashion.
The sniper is at his post near a military base. He sees a heavily loaded van approaching at high speed. The driver is ignoring the warning signs and shouts from the guards. The sniper shoots the diver. The truck is found to be loaded with explosives.
Did the sniper execute his target, or fire the first and only shot of a battle?
If not, then I would consider it an execution.