Both Endings-
Ending A-
When you get to the first floor, go through the door. Select "Go back". Grab the Kar98. Go back to the counter and select "No". Then select "Scare him". Follow him into the white room. Search the left wall for a gray outline of a rectangle. Click it. When it zooms in, click it again. Take the 3 bullets. Then go back to the main view of the white room. Go through the hole in the wall. Prepare to shoot.
Ending B-
When you get to the first floor, go through the door. Select "Go back". Grab the Kar98. Go back to the counter and select "No". Then select "Scare him". Follow him into the white room. Go through the hole in the wall.