Regarding Ephrim: Here's my take. Be aware that I'm reading a lot into Andi's character and motivations as a player and this could be off the mark, obviously; but in the sense of reading into Ephrim as a character within a fictional work, I stand by it.
So. Remember that Ephrim's alignment is Chaotic and is to 'Spread the dangerous idea that I am the only thing that can "stop the coming winter."' Ephrim has consistently placed faith in himself above most things, and in a sense his faith to Samothes was also a patting-his-own-back thing because the flame of Samothes also comes from within him, both narratively and systemically (the flame is affected by Constitution, i.e. his own body). Up until Ephrim met Samothes face-to-face, I'd believe it if you told me that Ephrim saw himself as an extension of Samothes' own will and body; that he'd believe the Prince's agency is Samothes' word.
Thus: The idea that Arrell does such profane acts also in an attempt to "save the world" – to "stop the coming winter" – is not only unjust, but almost a personal affront to Ephrim's identity. 'It's offensive to witness someone try to pursue my goals while preventing me from doing the same,' so to speak. I don't mean to say that Ephrim is so arrogant; but that he embodies the righteous flame, which burns intensely and single-mindedly, and that Ephrim indeed fully believes in this ultimately condescending construction. This intense nature of a righteous fire really does, in my mind, make it believable that Ephrim would not give Arrell's alternate plan even a second of consideration. It is wrong and he is right.
That same idea, then, feeds directly into Ephrim's fatal betrayal of Samothes. Be aware that this isn't sudden – Ephrim did meet Samothes when the former died, and felt an intense reverence; but then Ephrim learned the story of Marielda and learned that the Samothes he worships doesn't necessarily stand for the same things Ephrim has devoted himself to. And, as we saw, Samothes asked Ephrim to buy into an untrustworthy plan ('destroy the ancient blade that you've devoted yourself to preserving') just moments after Arrell practically asked Ephraim to do the same sort of thing. I think this is what justifies Ephraim exclaiming that he's "tired of being someone's tool." His instinct is still a desire to be the only thing that can stop the coming winter, the only thing that can be relied upon to be just, and up until recently this was an extension of Samothes – but now Ephrim has internalized that Samothes' will is no longer aligned with his will. Now Ephrim has plenty of reason to distance himself from Samothes and fall back on what he knows (the internal righteous flame). Now Ephrim has taken the opportunity to become his own King. And that is how you portray a character that is literally Chaotic Neutral fire in-the-flesh. Eh?
Anyway, what's been fantastic this season is how both Ephrim's and Hadrian's character arcs end up reflecting very different approaches to faith. They're both devoutly religious characters, but Ephrim is the fire and Hadrian is the rock, and I love how this show has remained consistent with that and rewarded everyone by following both paths to super interesting, super honest conclusions!