There is no solid evidence that Setskever instigated the firing beyond speculation by friends who suggest that he had disagreements with Altman. It could just as well have been any of the other board members, or even a simple case of groupthink (the Asch conformity effect) run amok.
Furthermore, it's consistent with all available information that they would prefer to continue without Sam, but they would rather have Sam than lose the company, and now that Microsoft has put its foot down, they'd rather settle.