Started out one thing that got really long...
Was it really Dean that made Cas upset?
It is obvious what he’s doing is something he's not comfortable with. He regrets it (shown in epi 18 and preview to 20) and is piss off at Crowley for being so smug (showed in the ending of 19). So you have guilt and self conscious about his own actions. As someone pointed out, out of his millennia lifespan he only endure free will in just a few years. He is ridden in doubt and is clearly unsure what action he needs to make (preview to 20) in order to bring order into heaven and assure earth is safe.
He couldn’t even go to the brothers (or really Dean, maybe Bobby?) because he knew he was going against what they believed in. Heck, what he believed in. Perhaps all those times he said he didn't have time to help them because "of the war" wasn't the complete truth. Maybe it's because he didn't want to continually lie to them. He lied about Crowley being dead and honestly, apparently been giving reports to Crowley (after all, how did Crowley know where to send his beasties? Cas made a statement that Eve manage to hide herself from any angel, and by default that should extend to demons too).
So that in mind, bringing it to last night episode. He was obviously stress and then suddenly he was blocked by Eve with a single thought. Not only this is not going to be quick but he’s a fish outta the water. Just how much power does Eve have and just how much does she knows? Could she possibly know his ties with Crowley? Would explain his nervousness to suddenly look around while they were in the diner.
Perhaps his nervousness about Dean and Sam taking the kids back home is not them wasting time but more on the lines of them getting caught in the crossfire. Maybe he knew something or picked up on something about the kids. He’s a soldier first and seen war so he knew a hell lot better than most to assume anything as being safe. He could have been suspicious and those are usually enough for him to act (No mercy toward Jesse back in season five when he concluded the kid is the anti-christ).
Then it can also extends further seeing how it revealed in the episode, Crowley wants Eve gone. So no matter what outcome of their fight with Eve, Crowley will take up the rear in case they failed and have demons in waiting to annihilate any beastie that comes out of that town.
Either way it’s explains why Cas was determine and emphasize that Dean and Sam stay put. It is not so much for the greater cause and that they are wasting time but if his suspicion is true Dean and Sam might get killed by either Eve’s weapon or by Crowley’s clean up crew.
I take this route because out of all the characters, Cas is the one person who knows Dean through and through. He knows his strengths and his weakness and seeing how it was two orphan brothers in need of help must’ve send all alarms blaring in Cas’s head.
But he can’t say anything. He says his suspicion about the kids Dean would fight him and still leave. If he mentioned his real fear about the demons that may be in the town, then Eve would hear it and hell comes back on earth. So, he’s pretty much stuck between a rock and hard place.
So, he’s beside himself and is stuck watching Dean and Sam walk out with the boys. Who knows what may happen because he's at the spot where he doesn't know and that probably scared the crap out of him. It was a preventable risk and he knew that. Of course he's going to be upset. He was upset lying to the boys in the last episode, this situation is adding salt on an infected wound.
Here he is basically lying to the people that were there for him when he rebelled. He shouldn't have to lie to them or Dean (because heck that boy have some hell of insecurity issue...) He's upset and angry and probably more than eager in taking Bobby's offer in the interrogation. Even went as far as asking to take the wheels of said interrogation. He can let out his rage and sadness while getting information all the same...
And that’s my two cents of the situation...
But before ending this note, I throw this one out at ya: Is Balthazar an Angel?