re: Castiel seeing Dean as a “monster”, I think that:
1) This is indicative of the kind of person Dean has become and the kind of things he has done since he became a demon, and Cas can’t //believe// that this man could possibly be the same man as the one he risked everything for in s9 because everything that demon!Dean does is a demonstration of how he is just not a good person anymore
2) Castiel can’t reconcile this new Dean and so he believes that it //can’t// be Dean, that this is a demon who has taken control of Dean & corrupted him. It //can’t// be the same man
3) Misha’s assessment of Cas’ feelings are based on the first few scripts he’s read, and since we //know// that Cas is a complex character has multiple layers, we also //know// that it’s not a simple manner of Cas deciding, “Right, he’s a demon now, so he must be destroyed”. When it comes to Dean, Cas’ feelings aren’t so black & white. That’s what it might sound like on the surface, but once he’s past the shock and the anger he will undoubtedly change his position on the issue and come to grips with the idea that yes, //this is Dean and we can save him//
So, those are my thoughts.