Who would be found guilty of the murder of Drummond in a truly fair trial?

Milchick, as manager of the severed floor, failed to maintain appropriate Occupational Health & Safety standards by allowing a firearm and ammunition to be available and removable from the sacrifice chamber without adequate controls.

No one? Seems like it would fall under self defense. It was self defense and self preservation that made innie hold the gun on him, and the trigger was pulled on accident. If I were a lawyer I could probably find a relevant case.

It was a tragic elevator malfunction.

In some current jurisdictions, it would be manslaughter by the outie. He placed the weapon in a position that allowed the accidental discharge by the innie.