Does anyone realize the magnitude of the choice Cas actually made here?

There was every indication, from the way he reacted, that Anna’s plan was viable.  Take out Sam and it would all be over.  No Apocalypse.  No Lucifer.  The world would be safe.  Dean would be safe.  And maybe he’d get his sister back; the one he’d hunted, who tried to tell him the truth, and who he betrayed because he refused to hear until it was too late.  For her, and maybe for everyone.  The one who saved him from Uriel.  Who’d trusted him.  Every other angel out there was under orders to kill him, and here was Anna, asking him to help her save the world, in spite of what he’d done to her.

But Sam Winchester, the boy with the demon blood, Lucifer’s vessel, was his friend.  And Sam meant everything to Dean.  So Cas sent her away. 

And why did Anna go?  Why not fight it out right then?  Her power was probably still depleted, but so was his.  He had a weapon that could kill her, but there was no guarantee he’d be able to keep it.  She was his superior.  She might have won.  The risk couldn’t have been so much greater than a reckless flight through time, to an era where any angelic presence on Earth would stand out like a beacon, to attack the one family on Earth that was under constant surveillance.

Anna left because she didn’t want to fight Castiel.

Back in Lawrence thirty years prior, the boys left him helpless in the hotel room, and they didn’t know how to angel-proof anything at that time.  Anna surely would have sensed his arrival, and once she saw Sam and Dean were there—without him—she’d have known how badly the trip, with passengers, must have messed him up.  She could easily have found and taken him out…if she had any desire to do so.  But he made it home in one piece.

These two were robbed.  There was so much going on with them that was only hinted at, and their story was cut short much too soon.