I have seen some antagonist being harder to convince and providing good counter-arguments, and would have wanted more for mister Greyt, but the Master reaction makes sense in context. The guy was emotionally driven to cope with the trauma of humanity and his own trauma. If not for having a goal through the unity, the master would probably had killed himself or went feral, after his horrible mutation. The unity was his way to cope with that, give himself a focus. Then, in the name of the unity, he sacrified a lot of people. So, when that fall apart, he has to face his own condition AND the guilt for all the people he sacrified in the name of the unity. Also, the unity higher ranks probably anticipated he would react like that. They don't lie to him for nothing.