In Resident Evil 0, Wesker and William are busy behind the scenes as they attempt to cover up the virus outbreak.
Unlike Wesker, William has no intention of leaving Umbrella at this time and wishes to continue his research, even if it means destroying the Training Facility.
Still, the way he eventually abandons the Training Facility without a second thought suggests that his actions are dictated by his own personal ambition rather than out of loyalty to Umbrella.
The camaraderie between these two seems to be honest enough, even though they are the bad guys...
Date: Mar 31 2016