Okay, let's recap a little what I saw and heard:

Sam&Dean made a blood pact with Billie to die but one of the Winchester's dies at midnight.

So Billie agrees, they kill themselves and come out alive. Then midnight strikes and Billie comes and is like

And then Cas is fucking sad and in shock and can't believe they did this and Mary offers to sacrifice her own life to save her sons' and right before she shoots herself Cas stabs Billie with the angel blade?! He is like

And then he says the most heart-wrenching shit ever like omg

I was crying so hard.. Let me quote them wait:

Dean: *shocked* "Cas, what have you done?"

Cas: "What had to be done. You know this world? This sad, doomed, little world? It needs you. It needs every last Winchester it can get and I will not let you die" *looks at Dean now* "I won't let any of you die." *keeps looking at Dean intensely* "And I won't let you sacrifice yourselves." *looks at Mary and Sam* "You mean too much to me. To everything."

Sam: *looks like he could cry his heart out*

Dean: *is sad and also happy that he feels this way; that he stopped his mom from killing herself*

Mary: *is surprised and confused*

Cas: "Yeah, you made a deal. You made a stupid deal and I broke it."

Sam: *looks ashamed*

Dean: *looks ashamed too; also kind of sad*

Cas: "Yeah, you're welcome." *looks at Dean intensely again*

Mary: *turns around and looks at Dean, too*

Dean: *looks at his mom at the same time as she looks at him*

•••

Erm... Mary just realized that Cas loves Dean. She JUST realized that Dean means a lot to Cas and she can't let anyone of his family go. For his sake. For his happiness.

This is Cas' way of saying:

And he damn well hit it right in the feels.