Sloor's plan is to let them come in, look left, see the "wall", look right, see the bodies and move in to investigate (or just turn that way would be enough). TB triggers the door with his telescopic arm, we quietly slaughter them with the help of surprise and we're good -- until somebody notices them missing from the front door, that is... Sound good?
Firing right as they enter is a sure fire way to get even more troopers coming in, I think (a lot of noise).