Castiel knew that Sam thought that he fucked it all up, that was for Sam the reason that Dean could hit him and yell at him. Sam thought he deserved it. That there was no way to redeem himself. Castiel knew better.
Castiel saw that Sam was struggling with himself, the dream visits of Lucifer and the burst outs of his brother. Sam was on breaking point, but that was he by the idea of Castiel all a few months. And still he didn't broke.
When Sam and Dean went to bed he heard Sam say something very quietly and he had to listen very carefully or otherwise he wouldn't hear it. Sam did every night a little prayer, even if Sam thought he couldn't redeem himself he could at least try to redeem himself as much as possible.
So every time before he went to sleep he prayed for him and Dean and said:
Now he lay there down to sleep,
I pray to the Lord his soul to keep,
If he may die before he wake,
I pray for the Lord his soul to take.
Castiel knew that he meant it was for Dean, Sam never thought these days about himself. So it was no surprise when Sam went thinner and got dark cringes under his eyes. Castiel also heard that whenever he woke up from a dream of Lucifer, he went to the bathroom throwing up. Today it was so day, because Sam run as quietly as possible to the bathroom.
This time Castiel Followed him. He saw Sam against sitting the bathroom wall and saw a few tears coming from his eyes. When Castiel said nothing Sam said " You don't have to come Cas, I'm fine. Just a nightmare right? " Sam hesitated. " Yes Samuel, just a nightmare so try and go back to sleep okay?" "Yes I will Cas, thanks" Sam said and then went up to go to bed, but before he went he said "Sorry Cas that I caused so much problems. You guys were all right, I'm an abomination. Night Cas" and he went in his bed and immediately dozed off.
Castiel still stood in the bathroom with his mouth slightly opened. Sam said the angels where right? Since when was that? He knew since last month that he thought that everything that was happening now was his fault, but this? He had to speak Dean tomorrow, but first he had to do some important stuff.
The next day when Castiel came back he saw Sam laying on the ground unconsciously and he looked like he took a good beating. But he didn't saw Dean. He laid Sam on the bed and looked outside. He saw that the Impala wasn't there, then where was Dean if Sam was in so bad shape. So after he had one last check on Sam he went to find Dean and only to find him in a local bar.
"Dean why aren't you with Sam? Your brother isn't in a good shape he looks like he is beating senseless." "Yeah I know Cas, I did it. You know why, if he didn't go away with Ruby then Lucifer would still be in the freaking Cage, but nooo why listen to your brother if you can suck blood from a demon. So yeah I hit him, so can I drink further? Thank you already bye Cas." And Dean went to drink more. " You made a very big mistake Dean Wincester." And with that Castiel left Dean and went to check on Sam.