The scream is pure humanity. Anna as she has become, not the Anna who was.

Castiel has been met with fear from humans before. It is not for nothing that the words, Be no afraid, have been attributed to angels.

In the course of his duties, he has used that fear to carry out his Father's purposes. And he has dealt death and destruction, also in the line of duty. With resolve and efficiency.

But that was different. This? What he is feeling now?

This is wrath.

And so when he retreats to the far corner of the room, it's as much to protect himself as it is to spare her further fear.