Incompetent engineer arrives to work late, botches an experiment, and ends up ripping a hole in the universe to a world full of "creatures". He then mercilessly murders the scared creatures while sabotaging efforts by security forces, and an elderly man in a suit, to resolve the situation.
!The engineer would be Gordon Freeman, the creatures would be the enemies from Xen, the security forces would be the soldiers, and the elderly man in a suit would of course be the G-Man!!<