Holly’s hit the nail on the head here.
Because this is Cas trying to be, to feel like, S4 Cas (and, also, remember, he’s speaking to Soulless Sam, and Castiel himself thinks Soulless Sam is about as much Sam as Dean thought Godstiel was Cas [and honestly, Soulless Sam in S6 is probably about as much of a confusing and potentially dangerous wildcard he’s wary of as Dean Winchester, the faithless, vulgar, confusingly unfitting Righteous Man, likely was in S4]. He figures saving him is off the table, and is very wary of how dangerous he could be; he treats Sam as the same kind of abomination whose salvation is impossible as he does demons—humans, by Supernatural's definition, are not soulless, and deserve his respect and protection [against all odds, he's learned], but right then, Sam is not of that kind by his estimation, and so Castiel, particularly in the midst of trying to assert to himself and everybody else that nobody can get the better of him [and oh, what a season-long theme that is, whether it be Balthazar, Rachel, Atropos, Dean, Soulless Sam, Sam, Meg, Crowley, or Raphael], is unwilling to take that kind of shit from whatever Soulless Sam is).
Season Six—and this episode especially—was very much an identity crisis for Castiel, just like Four was one for Sam (when he was out of under the control of/not beholden to Dean, just like Cas, had a Big Bad who’s outlandishly dangerous, just like Cas, and had someone else to step in when reconciliation of his identity with his new situation became difficult, just like Cas—oh temptation arcs, oh yes).
For Cas, he’s finally been out of Heaven long enough, found his touchstone, to the point that he can’t really be reset easily (the way we must presume Anna and others have been, and the way we know Castiel has) from his love of humanity, his compassion for their plight, and his love of Free Will for not only God’s most favored children (humanity) but for himself (and other angels). He’s been out of the abusive situation that demands nothing but compliance (no matter to what) long enough for some of himself to return, to deconstruct all the violent, compassionless, obedience he has been abused into for millennia (and then allow him to choose when he uses it, what level of force is acceptable by his own ruler rather than that of the cruel dicks with wings who decided for him before).
S4 is Castiel’s tentative first breaks from Heaven that keep being thwarted as he keeps being reset. S5 is Castiel’s efforts to survive and cope with being away from Heaven and making his own choices, being exposed to humanity in a more individual way than he ever has before as a silent, protective overseer of an angel (and trying to fight battles without Heaven telling him what to do and how to do it and why). S6 is then how he tries to reconcile what he’s learned, what he’s become—which is what he really is, without being tortured into being something else (the harsher bits of S4 Cas you’re mentioning, Holly) all the time, somewhat—with a Heaven that is unprepared for and not accepting of that, with the power over Heaven he has been given and yet the fact that that means he now has more of an obligation to a family that does not accept the kind of person he is than ever, and as a bonus, with human friends who he is no longer beholden to for any reason based upon utility. It is a fight for agency and it is an identity crisis; Castiel’s crack in his chassis has finally become sufficiently irreparable, and trying to be what Dean and Sam need because they’re his friends without Heaven telling him he must, plus trying to be what Heaven needs, considering he and Heaven see eye to eye on almost nothing and the things Heaven wants endanger the former, plus trying to understand what he needs as well as what he should do? It’s enough for anyone to lose themselves a little bit.
When he says, “Much of the time, I would rather be here.”, I think a lot of what he means has to do with the fact that he’s learned it’s much easier to fulfill the “human”, as well as relatively mild, power-related expectations Dean and Sam have of him (such as, for instance, responding to the forced kiss of someone who’s wearing a pretty actress/simultaneously dominating a demon to show both his ability to ~human sexuality~ and his ability to cow any demon using any method with which they approach) than it is to return to Heaven where he’s literally fighting for his life and leading the parts of the Host who agree with him in a different way than ever before, without superiors, or deal with the cognitively dissonant ethical complications of working with Crowley out of seemingly desperate necessity as the leader of Heaven and literally the first and potentially only defense standing between the political chaos in Heaven and the possible destruction of humanity (which will accompany his loss in Heaven).
When he says “We’re not supposed to talk about it.” and “I learned that from the pizza man.” it’s a verbal nudge that even if he cannot be human like he was for Dean and Sam before, part of the team that way, he does learn from what their culture expects of him. When he says “Why are we working with these abominations?” and “Will you? Boy? How?” it’s a reminder to whoever he’s speaking to/with that even if he can be part of their team, he’s still an angel, with attendant mental predisposition to hatred of demons and distaste for being threatened by beings with inferior strength. (This is part of his crisis, asserting his formidably dangerous identity of his own power even as he’s not quite sure he wants to be that kind of person, and part of him would like to not have the responsibility that comes with such power and inbuilt hatred.)
It’s funny how often this GIFset is singled out, without seeing the context, which includes Soulless Sam tricking Castiel down from Heaven (where he was mid-battle) with the plot of a movie as a dire Heavenly weapon situation, mocking Castiel’s eagerness to help (among his desperation for more weapons to win the war) and his lack of pop culture knowledge, and then, of course, threatening him when Castiel doesn’t want to do what he has called him for (which is essentially to be a flyswatter for the demons with whom the Winchesters have agreed to work, Meg primarily, and her minions). “We’re bringing insurance,” Soulless Sam says, and then we see him calling Castiel for literally no other purpose but to follow them around and make sure the demons don’t get them. It’s rather contradictory, isn’t it, to call an angel down because demons—who’re even more powerful than humans—can’t hurt them and then threaten to kill said angel if he disobeys?
Castiel is aware of the differences in power between angels and other things all too well, right then, considering he’s spending all his time fighting the former and basically considers coming down for the latter a frivolous vacation to spend time with his friends (one he does not have time for). You can see on his face that it’s a bitterly amusing pill to swallow, being threatened by a “human” (who doesn’t create the same moral dilemma Sam does of killing his human friend—killing any human, but especially Sam, his friend—because he is quite resigned to Sam being gone, but is possessed of the limited power of a human compared to an angel) who called for his help when that particular expectation of his person is usually the least dangerous.
Basically, if there’s anything Cas was in S4, it was well assured of his own power (often naively so, and additionally confident of his own dispensability, which aided this), whether it was over other angels, or demons, or other supernatural creatures, or humans; he has since learned that it’s not that simple, but sometimes he attempts to step back into that role. And as Holly says, that’s what he’s doing here.
S4 Cas starts out with something the Cas of other seasons doesn’t have (and even then, not always): a comforting self-assurance, confidence of his own strength and skill, his absolute convictions, and his sense of self, because they were almost entirely determined by Heaven, by who Heaven demanded he be and manipulated him into being as a soldier, and he had no agency in affecting them, so he had no fear of failure. S6 Cas really wants, even needs, to feel that, dealing with such a precarious situation in Heaven (and trying to prevent it from spilling onto Earth) so you see a lot of this kind of posturing, as though he is again S4 Cas, only allowed to ever sincerely question who he is and what he can or should do in secret because those things cannot affect the fact that he is an angel with a sense of direly important purpose—nothing will, if he can help it (though of course we find out in S4, he can’t).
I agree, by the way, with you, Holly, when you say it feels wrong. The closest term I can think of to what Castiel is doing here—what he does in the entire episode— is self-objectification as a tool of righteous fury (so angel, very power, much cold tactical maneuver) and protection of his human charges, and that’s nothing good (though it can be helpful). In fact, my guess is it’s mostly remnants of how he has to function in Heaven, for his survival and as an infallibly competent figurehead and leader, inevitably leaking into how he acts on Earth, especially with the Winchesters treating him as a tool, relying so much on his power and knowledge and not really giving much of a fuck about his companionship (as he gauges it, though obviously we know they’re only leaving him be most of the time because they know how busy he is, only calling when they can’t do without him).
