They're Scared of the Light

Controls

WASD, Mouse - FPS movement
E - Interact
1 & 2 - Weapons (when you find them)
Q - Mirror (When you find it)

To get rid of the cursor at the beginning of the game, Alt + tab and then Alt + tab back into the game.

Find the Lamp or Flashlight to kill the monsters.
Shine your light on the monsters with the Lamp or Flashlight to kill them and collect their demon souls.
Ignore the note that tells you the types of monsters, there is only one type.
You may kill the monsters without even knowing.
If you can't open any of the Demon Soul Doors, just walk around with your lamp until the monsters spawn
It doesn't seem like you can die.
You win when you enter through the door in the biggest room (requires 2 demon souls)
It says "You Win :)" but doesn't actually do anything besides that.
To exit the game press Alt+F4