I suppose the rules of war. As long as an enemy is a threat, he will be fought. If it happens the enemy survives, he is no combatant anymore but a prisoner of war and has to be treated like one, as long as he is not resisting or trying to run away.
Boelcke was about the same. Enemy was disarmed, actually giving up (pilot wounded, observer was waving with his arms, clearly showing they were giving up etc.) but he thought to himself that war is war, and as much as he hated to do it, he still continued the attack until the enemy broke apart and crashed, killing both inmates.
Actually all aces were the same. Mccudden (we are murderers), Mannock, Rhys Davis (e.G. it was clear that Voss was wounded or worse) and still he went after him, strafing downed pilots etc. There are for sure tons of examples that downing the enemy, no matter what, was common. Also one didn't want to get tricked by playing wounded and then running away.
For sure in MvR's case there also a psychological factor. Him being shot at from the ground from the downed observer. Getting wounded although he thought he was in a save distance, or maybe he never actually knew who hit him from where. So as long as an enemy is aloft and somewhat intact (even when gliding one might pull out a pistol), he is a threat to his own life. So to make sure the enemy has to break apart or burn. To many ifs if an enemy lands intact. Will they try to run away, will they shoot at him from the ground, will they shoot at oncoming soldiers. Will they be able to run away and fly the next day...So down him to be sure, job is done.