I think the defining moment was when Bill turned on Ham personally, not so much for the actual attack but for the message which needs to be hammered into the heasds of fundy literalists: that their bronze-age beliefs are in danger of stunting the future of society.
At the age of five, Skagra decided emphatically that God did not exist. This revelation tends to make most people in the universe who have it react in one of two ways - with relief or with despair. Only Skagra responded to it by thinking, 'Wait a second. That means there's a situation vacant.'