Basically David brevik said that there was no story and the cut scene on the end was added by blizzard south. They had no intent to add that.

I stand corrected though. Metzen is credited as a writer on Diablo. But not Diablo 2.

I suspect Metzen wrote the ending that was tacked on according to Brevik, since at that time blizzard north was a completely different team at the time.