I suspect the Captain did a small risk assessment in his head and reassured himself that the First Officer was of sound mind and body before reaching for a half time tea. Out of the millions of flights that operate globally each year, the incidents of one of the flightcrew going rogue are remarkably rare. I wouldn't be unduly concerned.
I wasn't overly concerned. I doubt there was a third person on a jumpseat, given the route I was on (TLV - MAN) and my view of the cockpit as I entered and left the plane. As you say the number of incidents of mass murder whilst the pilot commits suicide are thankfully rare and my query was not one of accusation merely of curiosity. If the blanket ban on single cockpit occupancy is not actually a blanket ban than that's really all I asked to know.