Angels are sensitive to dark and evil forces that can be seen or unseen.
It was some sort of a gift carried by their genetics, the code that the Angel Origin, branded on them to know who to trust and help. Of course—it could only be used as a surface-level assessment but it's still helpful nevertheless.
Seeing the darkness Rex exuded was surprising but wasn't that surprising.
Eryndor had lived for a very long time.
Both Catherine and Radiel are considered as babies in his view.
So he knew that anyone who rose into power—as much as the position of an Emperor would certainly have darkness around them, becoming strong was only reserved for those who are willing to do anything—that's the reality of the world.
However, the darkness coming from Rex intensified as soon as he heard about Nadia.
Now, it was akin to a blank ink poured onto a blank sheet of paper.