dafeeeeeeeeeesh
The soldiers are seemingly unnaffected by the black smoke, it seems that their suits are fully enclosed, maybe that's why they weren't affected by the purple haze as well, however, it does affect the ones that are knocked out. Seeing that the bodies of those knocked out aren't worthy anymore, they neutralize those who they find when going back to the ship with a single assault rifle round to the head, and move on to extract from the world, pulling back all their forces to the ships.