Morok had been grieving as much as Lith, blaming himself for quitting the army and losing the right to attend the gala.
The young Tyrant also blamed himself for being too weak, for being non-Awakened, and for letting the second person on Mogar he truly cared about die a dog's death.
So, once Deirus had appeared, spreading salt on his wounds with mockery on top, Morok had lost it.
The good thing about being an Emperor Beast was that all Tyrants looked identical to humans. The good thing about having no tie anymore with the Kingdom was that even if someone recognized him, he had nothing to lose.
Alas, his strength and fury were nothing compared to strategy and planning ahead. Deirus had prepared his defenses so that they could handle Lith and Orion at the same time, let alone an angry beast.