jimworcs
The Kingair door is not a plug door. The door is supported open by steel cable on either side of the door, which double as a hand-hold.
The door is held closed by 4 barrels that rotate around lugs in the door frame when the locking mechanism is actuated. Whether one could open them when the aircraft is pressurised, I couldn't say.
On the 737, A320 and most if not all transport jets, the exit doors are plug doors. They're larger than the door opening, and generally open inwards to some extent, before being mechanically positioned so that they can fit through the smaller opening. Thus, with the pressure differential of anything from 6 psi or more, the pressure forcing a plug exit on its frame is many hundreds of pounds (6+ psi x area of door), and prevents opening whilst the aircraft is pressurised.
HOWEVER, for those quick to criticise the victim, the Kingair has been known for the poor door locking & warning mechanism, such that when one shuts the door, if one checked the position of the locking barrels against the lugs, one can ignore a subsequent door warning.
Kingair doors have also been known to open in flight. One famous incident, circa 1980's (I think), saw a Kingair Captain investigate a door warning in flight, leaving the Co-pilot at the controls. The door opened, the Captain went out, and was found once landed, hanging upside down with a death-grip on the lower door supports.
From this thread, it's not possible to say for sure, if the victim opened the door, or, perhaps, it opened on him, and he wasn't as lucky as the aforementioned Captain.