Although I do think Drift believed in the Decepticon cause, at least during when he first arrived at New Crystal City, I think the majority of his anger came from self-doubt. Sure, he was probably (or rather definitely) angry at the Circle for keeping him imprisoned in the City, but I think seeing New Crystal City in all its perfect glory made him angry.
Think about it: all these years, the Decepticons hadnāt achieved anything, not really. Sure, during the beginning of the Cause, they destroyed the Senate and functionism, but after that? All they did was spiral down into a tyrannical group who only wished to conquer Cybertron. Half of them didnāt even join for the original cause, they saw the violent nature and decided they wanted in.
Now, put yourself in Drifts shoes. All this time, this future that he and Gasket had dreamed of, the future that he joined the Decepticons for, had slowly slipped out of his grasp in the wake of Megatrons insanity. Now, suddenly, youāve been dragged to this City, this perfect utopia, where everyone is equal, thereās no suffering, no one is corruptā¦
This City, that even mecha who didnāt even join in the war, didnāt believe in either cause, had managed to build behind their back? Constructed the perfect society without violence, without an uprising, without a war? It was a harsh wake up call.
Because, if this paradise, this fantasy, could be achieved by peaceful means in little time, what was Drift truly fighting for? Familiarity? Purpose? Heās not had a purpose for a very, very long time, and if he did, it had been lost many years ago.
Heād been one of the top Decepticons for so long, being feared by his fellow āCons and Autobots alike, had a reputation at killing them, and he was good at it. He enjoyed it. It gave him a sense of false purpose whilst he was slowly losing himself.
Now, heās imprisoned in a paradise, a City heād only dreamed about, flaunting about its greatness in his face? Heād been humbled multiple times by being less skilled by other Knights, no longer is he feared, heās now on the bottom of the barrel once more (whether that was intentional or not doesnāt matter).
What else could he do but lash out? Deny every piece of the City because he refused to acknowledge that maybe, just maybe, heād made the wrong choice? Placed his trust in the wrong person, spiralled down a path of violence and anger from placing said trust in said person?
Edit: I realised after writing this that Iād sort of said some of the same points already in this post, but theyāre also both slightly different. I also expanded on that post slightly here, went into much more detail, but just thought Iād mention it :)