“He should’ve believed in the boys to fix everything”
The boys have almost always relied on the help of antagonists/villains or at least people they never would’ve worked with otherwise if the situation hadn’t been so dire to solve their problems. Cas is only doing what they’ve always done themselves.
Also, is it really that hard to understand? Like, really? Really? Amara is presented as being a bigger threat than Lucifer and Lucifer is a means to stop her. The enemy of my enemy is my friend, as they say. Not to mention how they’ve already stopped Lucifer once before and at least there are multiple known ways in which to subdue and kill Lucifer, unlike Amara.
and let me just add that the fate of the world was already at risk with Amara being free who as stated above is a much bigger threat than Lucifer. Hell yeah Castiel was scared and for good reason. Heaven threw everything they had at Amara and she shrugged it off (okay, it seemed like she might of been a little hurt, but she showed none of that weakness while Cas was present so he wouldn’t know). Amara’s crazy powerful, God’s older sister for fucks sake, and they have no idea how to kill or even contain her.
Lucifer said he could defeat the Darkness, as an archangel he’s an incredibly powerful being, and he has fought the Darkness before. It’s not ideal but really he was the best chance that has been presented so far. Castiel sacrificed his autonomy and risked his life to do what he felt was necessary to give them even a sliver of a chance against Amara.
Also he didn’t go behind the boys back. Saying yes to Lucifer was not a premeditated decision. It was something Castiel decided to do during the few moments he spoke to Lucifer in the cage. Sam and Dean didn’t hear, that wasn’t something Cas planned it’s just how things worked out. Lucifer was the one who decided to pretend to be Castiel and keep him being free a secret.
Maybe if the boys showed more consistent care about Castiel’s well being they would of paid more attention to him in the cage and heard his discussion with Lucifer and him saying yes. Maybe they also would of seen through Lucifer pretending to be Cas instead of needing the archangel to out himself for them to get a clue. Sam and Dean taking so long to figure out Lucifer was possessing Cas had nothing to do with Castiel keeping secrets it was them being shitty friends and not realizing something was off about him/not caring enough to look into what was wrong if they did notice.
“He should’ve believed in the boys to fix everything” lmao They’ve never fixed anything without his help and the one time they could have helped him aka Season 6 they refused and tried to prevent him from saving the world. If he believed in them to do anything without him the world would have been over since Season 4.