Frantically investigate the gun more

David stared in horror at Seth's lifeless body on the ground. The final words of Seth echoing in David's head.

"Why?"

David quickly averted his gaze back to the gun. Surely if this is what caused this madness to happen, it could revert it too! Only problem was that it didn't come with any instructions. For better or worse though, there were only a few things David could do. There was a trigger that he just used, some arrow buttons on the side, pointing left and right, and a large red button on top. There also seemed to be some sort of hatch that he could open just below the large button.