He had no fear of her and he certainly wasn't going to back down, even physically from this position. She had not been smart about this. But he had to struggle to stay focused. At first his body reacted automatically to the nearness of her, expectations overriding the situation briefly and then he had to struggle with her words. A part of the reason he was so emotional to begin with had to do with his own sensitivity. Because buried underneath was the simple truth that she had preferred to risk herself completely on a stranger than rely on him even a little. Nearly every word she spoke seemed barbed specifically for all the right tender wounds he had worked to seal away and she just cut right in. And for a moment he could have lost himself to the hurt, except for all the truth she spoke with she certainly wasn't acting as though she knew how to protect herself.
"You let a man who can perform spells unlike anything we have ever seen, that you barely know, come into your space and blindly trusted he meant you no harm and now you tell me you know how to protect yourself? That was not self preservation. I do not have to be involved in those decisions, but I do expect you to protect yourself when you make those decisions. I expect you to use everything you have at your disposal."
The knowledge that Severus had been the one to set her off on the run did not raise his appreciation for the man. No, to Castiel it seemed more like Severus had used a minor situation as an exploit to gain her trust, because despite the impression that Meg had, no one was about to knock on her door and douse her with holy water. Basically no one cared they were trapped here with a demon, except Jo. Things wouldn't be critical until someone was hurt but she would never see that. Just as he would never understand how Dean could forgive him every single mistake but not stand up for her even though she had risked and lost everything for Sam. "You think I would ever let them anywhere near you if they meant you harm? I have never intended to let you come to any harm, especially at their hands. I will not allow it."