Here is my hypothesis:
It was totally dark in the room except for possibly one bulb at the top of the Buddha lamp. You can see that the lamp is off because the bottom bulb is off. Maybe it was burned out. But, still, it wouldn't have given much light at all in the room.
Alcala (for whatever reason) has seen this girl before...followed her or maybe is a neighbor. He stakes himself outside her window with his trusty camera. He is either able to see into the room perhaps from the very low light of the upper bulb in the lamp.
When he sees her cross the room, he takes his shot. You can see the tiny flash on the right side of the photo. The heavy shadowing of the room simply means that when he flashed, it lit her up but cast her deep shadow on the walls. I think that the object on the lower right corner is a bush next to the outside of another window perhaps.
The only thing on her cheek is her hair casting shadows. She could have the line on her body from either a scar or from laying on a towel or clothing she had on top of the bikini.
I just don't see anything evil about this photo other than a serial murderer peeping at her - which in itself is terrifying. And, who knows if he later didn't kill this girl. But, in this photo, I see nothing but a girl walking across the room - drunk or loaded perhaps. I think she looks like she doesn't even know anyone is around but her and then a flash comes, thus the dazed look.
I just don't believe that in these photos that except for 2 of them, there is any kind of knowledge of anything evil happening to them. Think about it, killing the way that Alcala did was a very intimate act. He strangled them...the most intimate way you can murder someone. I'm sure that by this time he had them naked. He loved naked women. This woman seems totally unaware of anything going on. That's just my opinion and can't say that I have tested shooting anyone in a house from the outside at night. Not a peeper!