The error was the point where all the alarm bells went off - and they decided to keep going.
CJ, stabilized approach criteria and non adherence to EGPWS have already been
mentioned. The error(s) had probably already been made , these are safety nets that failed, question is why?
.... asking for the short-cut to runway 21 probably seemed like a completely reasonable request at the time. They were tired.....
Reasonable? Quite the opposite!
Asking for a shortcut when tired, that's more like the error to me.
At the very least it is a threat that one should be aware of.