Anger mixed with anguish from the devastating realization makes Rex shout like a maniac.
Evelyn and Adhara were stunned when the two saw this as they can't imagine how much restraint Rex has to actually be angry and pull himself together at the same time. Even though his anger can be seen depicted in his eyes, his aura didn't explode and went rampant.
Knowing that doing so would expose the castle's location, he restrains himself from doing that.
Hence his outburst was pure without using any kind of power at all, like an ordinary man cursing at the ghosts from the darkness that hides in the shadows. But because of this, the anger in his eyes can be seen more than usual.
If eyes can kill and cause destruction, the entire place would've exploded into nothingness.
