Rex has been nothing less than a man of action throughout his entire life.
He speaks more with his actions rather than his words.
Despite the taunt and mockeries from his arrogant enemies, he rarely responds if he is on the back foot—knowing that doing so, would only make him look ridiculous. Almost always, when he does speak, it's when the situation is fixed in his favor.
But now, he did the complete opposite.
As of this moment, Rex was on the back foot, completely outclassed by the chaos monster.
Nevertheless, he threw vicious words and promises at it.
Frankly speaking, by now, it was not a rare sight to see Rex becoming berserk and consumed by the anger that most of the time—lie dormant inside of him. Every time that haunting anger comes out, it has always been a terrifying sight to see.
However, the current sight scares the others more than the past rampages Rex had done.
Rex was unable to vent through his actions.