They all believed the waypoint was at McMurdo Station, but thought that a route to that point would take them down the Sound with Erebus to the left.
That's the nub of the thing, right there.
It is what was in the heads of the crew which led to disaster.
That mental model had been set up by the airline and to a lesser extent by the CAD. That is why the prime responsibility rests with the airline management and its overseer.
The fact that the crew members were hopelessly inexperienced in the task at hand and their training was woefully inadequate compounds the culpability.