I think the killer worked in near darkness and that the victims were killed where they were found. He only had to be able to see what was within a couple of feet of him after all. There was also the added advantage of his own near invisibility. By that I mean that it's much harder to see someone when you are looking from light into darkness than it is to see someone when looking from darkness into light. If the killer can somehow manage to work in a dark place then anyone approaching, even if from somewhere which is only slightly lighter, will be more clearly visible to him than he is to them.