Hang on. The Captain leaves the flightdeck to a comfort break. When he returns he finds the F.O. has locked him out. Too damn right I'd be worried about terrorists. I'd say the captain's initial reaction of banging on the door and shouting to be let in was perfectly normal. Seems everything escalated from there. I'd love to be in on the investigation; not sure the F.O. has covered himself in glory here.