1/7 - Go inside the cabin, where you get the first key. Jump on top of the table with the rusty saw, then jump on the shelves next to it. The hidden piece is on the top of the wooden wall.

2/7 - After getting the crowbar, look behind the mattress you just woke up and find a little cracked wall, break it, take the key, and open the room door.

3/7 - Take the key from one of the rooms at the brick corridor a zombie runs across, and back to the locked open the locked door next to the blood syringe door.

4/7 - Pick one of the boxes from the laundry room, bring it back to the same room you got the shotgun, and use it to climb the metal wall.

5/7 - There is a button under the desk at the control switch room, press it and it shall open the bloody door at the corridor.

6/7 - Right after the exit door there is a locked door that requires a key card to open it. The key card is right on the floor, hidden under a fake tile.

7/7 - While in hell, jump on the third platform, then look back. There's a button on the second platform. Press it and go back, a new path will appear (I can't figure out the code. Someone posted it being 26-16-4-27-49-2-32-45-12-23. But I still can't open the door).

Thank you everyone in this discussion for helping out.