I think Rex was wrestling with himself but with a block. Rex has never hesitated for a moment before and he was resolved to do it despite knowing the consequences. The strain seemed as much physical as it was mental. Really he seemed to exhaust himself wrestling with it. Rex said so himself. For Dogma the breaking point was the friendly fire incident and for Cut the breaking point was losing his very own version of Domino squad in a gunship crash and watching survivors get euthanized. What is it going to take to break Rex like that? Maybe it was right then and there when Dogma fired the shot. I think a good argument can be made for Fives having been broken from it already, although he does everything he does in "Plan of Dissent" out of loyalty to the Republic, he really seems every bit as independent and free thinking to me as Cut it, he just uses it differently. I think that comes down to temperament. Fives is an angry SOB and he wants to break stuff.