Pretty sure he had more options than starve on Kirkwall's streets or be second-in-command to a tyrant. Off the top of my head he could request being sent to another Circle if he no longer wanted to serve under her.
Being a Templar is like being a clergyman, or being a member of the SEALS, or some exclusive hard-won membership in a prestigious fraternity or social organization. Cullen is a believer. He is an Andrastian, when the tower in Ferelden ran mad (and it DID run mad with 90 PERCENT of the mages I encountered either abominations or blood mages and the other mages DEAD on the floor), everything that he was taught to believe was his purpose was verified. Who was left in that tower that was sane? Wynne and one or two others? Cullen had a first-person, direct experience of just how ugly it can get when mages are demonized, literally. After Origins, he has NO reason to think well of mages. What is amazing is that he doesn't fall in with the abusers and kick mages about.
Is it any wonder it took him so long to stand up to Meredith? First of all, he has to overcome the group think, and if you believe that it is easy to oppose the group, particularly the group to which you have given your allegiance, then I would suggest that your ability to withstand group pressure may never have been tested, at least not at the level of serious consequences that opposing Meredith would have required. Also, as a quasi-military organization, I seriously doubt that Cullen could just say "transfer me" and have it done. Even asking for a transfer would have serious consequences, just ask anyone who is an officer in a military organization, and see what they have to say.
Over and over again various social experiments have shown how incredibly difficult it is to stand alone against former comrades and friends. Just think about the group think of high school cliques. You think it gets any better, later? Deciding to stand up against the status quo is difficult under the best and most liberal of circumstances. In the Circle in Kirkwall, standing against Meredith and her crew could probably get you killed. That he did so at the end was brave.
Finally, I love the holier than thou mages out there who condemn Cullen and cast him forever into the outer darkness but say nothing about the way they were sold out by their own leaders. And how many of the mages in Redcliffe who didn't want to be indentured to Tevinter spoke up and drew a line in the sand the way Cullen did? I spoke to many of them on my way out of town, and so many were unhappy but none left with me or sought to oppose Fiona. Talk about yielding to the group--
As a mage, I can forgive him because as a mage, Maker only knows what compromises, betrayals, secrets and lies I have had to do and say to stay alive and whole in my circle growing up. How many times did I have to go along with the group while there was a nagging thought in the back of my head that maybe what I was doing wasn't the right thing? Cullen and I are both human. We are not gods whatever people may believe, and being human means making mistakes and doing bad things sometimes, things we later regret. We may do them out of ignorance, or fear, or hate, or for selfish gain, or simply because we don't think them through. But as mage, I lived in a glass house. I learned not to throw stones at anyone.