I do wonder. In the interior of our house, 77% illumination still leaves the hallways pitch dark, the middle bedroom too. Back of house faces sky and west, so dim light in kitchen. But wouldn't the sleepers have lowered their blinds - to keep moonlight out?
Maybe not, maybe enough glow from charging electronics to see a little, once the eyes are accustomed to the dark. I now believe the killer found exactly the 4 people he thought he would find - perhaps not exactly where they could have been, but he probably knew E and X were in that second floor BR. I am thinking we will find they were killed after K and M, perhaps because they were slightly roused from sleep by some sound in the house.
I wonder if K and M had closed the door to their BR (I wouldn't, if the bathroom was outside the room and I'd been drinking, there was no one else up there or expected to be up there). But E and X probably shut their door due to proximity to the kitchen.