Hunter or not, Cas liked to believe he wasn't like Sam or Dean. They had their ways and methods - those were not his to criticize or judge. In days past, he would have smote first and asked questions later, but now he felt that Anna's approach was more...logical. Not all beings of a supernatural persuasion were monsters. Many were simply trying to survive. He may have threatened Phaedra in the beginning, but it was only because he himself had been presumptuous. And hurting. That didn't excuse it, but he was glad he hadn't simply acted.
"And you would?" he asked, genuinely confused.
While she was a vampire, he'd never taken her for anything other than direct. If she was ever furtive, it was likely due to displeasure rather than anything else, but that was fairly understandable.