If you're looking for some inspiration, I'd definitely recommend https://en.wikipedia.org/wiki/Nineteen_Eighty-Four
In this book, the protagonist must survive in a dystopian world, outwardly obeying the party line while in his heart feeling revulsion at everything they do.
Aside from the approach, the writing is excellent, and the story compelling. There's still a lot of room around this though to explore other areas.
What you describe sounds like a very similar dichotomy where someone's internal thoughts and external actions are at odds - I'd think you'll need to go for a first person, or certainly a zoomed in 3rd person style to really capture what's happening with that character.
- The battle of doubt in the system vs self-deception
- Their morals clashing strongly with the work they have to do to survive
Building up to the point where they're put in a situation where they have to choose between truly betraying their beliefs, or risk losing their life/liberty.
I'd suggest you build up slowly to the point where they have to kill - most of us have a strong innate revulsion at killing other people. That could even be the ideal point for the character to make that choice. Perhaps they can be present while those around them do it? To slowly have to move further and further in that direction?
So, your character joins the local police despite their somewhat brutal reputation, and is slowly exposed to and indoctrinated into their beliefs.
You can be somewhat subtle here - maybe some of the police believe that they're doing the right thing - that they're the thin line between order and chaos. Sure, there's a few bad apples (read: psychopaths) who give them a bad rep, but ultimately they're doing what needs to be done.
Maybe your character starts attending to some relatively routine matters - the same way any relative novice would. Then they get brought along on raids ("this guy is a drug dealer") etc, before moving on to more and more brutal work ("traitors", "traitor's families etc).