There's no such thing as a silly question, only a silly answer, so here's my two'penneth.
If you've chased away then forced downed a spotter plane on their side of the lines by all means kill 'em as they could still complete their report, some did, most didn't. If the enemy comes down in your lines killing him definetely is not cricket, unless he takes a pot at you of course.
Having said that some of our chaps believed that Huns taking to parachutes were fair game!