Cersi stormed off in the meeting. Had she not done that, PERHAPS an option C could have been agreed upon. Instead she chose to leave the meeting, then come up with her own scheme to sell everyone out. Everyone else was willing to try and make an alliance work. But Cersi only wanted things HER way. When she couldn't have it, she opted to just sell everyone out. Which is all well and dandy except for the fact that the people she is screwing over are the people trying to save all mankind.