Locking doors and oxygen removal would have been just a fun story. Halogen gas, at a concentration needed to disrupt a fire, is only mildly dangerous and you do

Locking doors and oxygen removal would have been just a fun story. Halogen gas, at a concentration needed to disrupt a fire, is only mildly dangerous and you don't need to remove the air when you are using halogen. The halogen does not work by displacement but actually disrupts the combustion process. No need to lock the doors.