I'm sorry to say but this game or demo is unplayable. I tried many settings that didn't change anything. In the beginning graphic glitches appeared on the walls non-stop and I have a really really strong PC.
Please don't get me wrong, the start is great and the potential is huge, but you still have a lot to do.
Unfortunately I had to end the game at the point where everything went dark. I somehow moved from light to light for over an hour. I somehow got through several doors without seeing anything at all, but nothing happened anymore, as if I was stuck in a loop and have to Alt+F4 the Game because the ESC button wasn't working anymore.
I really hope you get the game fixed, I have it on the wishlist on Steam.
Best regards