It's the Dedication thing. The big old burst of belief energy you get just from mortals dying and releasing their soul energy. The idea is that the Goblins consecrating it to the memory of the Dark One meant he got enough Devotion (and the rest of the stuff from the Goblins who worshipped him) to ascend.
Or at least that was my understanding when I read the story of him ascending.
Depends on if enough Devotions would do it, or if you actually needed Worship and Belief first too. But the other point is that BY massacring enough people that it would propel him to Godhood, he would strike fear into the hearts of the rest of the world. Killing Azure City isn't going to do it. He needs to massacre like most of the civilized world and put himself far above other murder happy liches.
I mean the Dark one didn't get power because a bunch of Goblins worked themselves up into a frothing frenzy over him, it was because they wiped out almost all the Humans until the Elves and Dwarves stepped in to make there be peace. (Start of Darkness)
But if double crosses the Dark One to save the world, I'm not sure if he'll get that coveted position.