One of the accident experiences that I've been unfortunate enough to be involved in was attributed to an F/O going sick and being replaced at short notice by another Captain. The resultant accident occurred with them both concentrating on the instrument panel and no-one looking out the cockpit window. This had nothing to do with taking off from a taxiway but could offer a possible explanation.
And just to be even handed, I as an ex-ATC would never clear an aircraft for take-off that I hadn't checked either visually or by radar was entering or on the correct runway. So as it was dark, was the SMR working?
There is generally no single explanation for why these things happen but I'm inclined to think there are probably a few holes that have lined up in that Edam. Time for everyone to examine their culture and procedures and take the necessary measures to prevent a re-occurrence.
On the beach