Hoo boy, this was a good one! I'll be honest, I didn't realise immediately that there was a switch to flip in the cabin at first, so I spent a good chunk of time wandering around the landscape waiting to be pounced upon without even starting the game up (handy little note for anyone else thinking about diving into this!).
The thing that got me wound up the most about this game was the slow build-up with the atmosphere. The little sounds creeping up in the background, wondering if that suspicious rustle was something sneaking up behind me, the anticipation of imminent death... it worked so well!
The angels themselves behave wonderfully too, just as you would expect them to! I think I was still missing some of the cues as to when they're right behind you, but by the time I was done I was beyond caring, I just wanted to get the hell out and survive!
This is an excellent concept, and I really hope you get the support to make the other levels you were planning on doing. Perhaps the other levels could involve things other than just flipping switches. A maze perhaps? (OK, I just freaked myself out there, don't even think about doing a maze!)
Keep up the awesome work! =)