You're a horror movie psycho.
You've been taking out the innocents of hollywood for a long time, this is the real deal though.
This unfinished level... I mean building... is swarming with armed guards who can take you out in a second.
Stealth is key, make sure you aren't seen and try walking (hold SHIFT) so you don't make too much noise, that's a sure way to attract attention.
You can also use the red doorways to enter your secret passages and hiding places (how do you think we get everywhere we do?).
WASD to move, left click to attack.
Take out as many people as you can, good luck!