Swap the bullet
While in the room, approach the shotgun and, by pressing "Z" interact with it. Of course, you have to do this before the murderer takes it.
When you interact with it, you need to swap the bullet from the cartridge chamber with the blank cartridge you found.
Then, you'll need to enter the wardrobe (marked above) in order for the murderer not to pot you when he comes to pick up the shotgun.
Finishing the quest
If you've done everything properly, a scene will soon commence, in which Reginald manages to defend against the murderer.
Finally, the woman from the tutorial will appear and provide you with Reginald's mask.
Reginald will also repair your pocket watch, giving you more control over the game time.