Seth's reaction is mild. Irahl's answer is satisfactory, and much easier to parse than the mess Vincent had sent him. Is this explanation unsettling? Absolutely, but if Seth got upset about every existentially-challenging thing he learned about monsters, he'd have been out of a job a long time ago.

As it is, his attention drifts somewhere else as he balances this account against what he already knows, and he responds with a slight nod, but that's it.

"Have you ever been in a situation where you've lost control of your senses, or hurt people unwillingly because of it?"

Oh, and there's that extremely blunt and personal question asked with the same inflection as everything else.