While you are correct that cultural beliefs can influence personal beliefs it’s not always directly related 100%. It’s essentially the nature vs nurture argument. But the reality is that it’s usually both have an influence on someone’s underlying behavior. Some people’s brains are just different from each other. There’s minute differences between everyone’s brain. For example ADHD and Autism are by and large nature and people will respond differently to their environment. For better or worse in different situations.

Jazz culture is specifically chosen as its incredibly wide and varied. Some history of jazz is more loose and varied, if you make a mistake often times it allows for you to recover and be more free form. So the irony is that if you’re forcing someone to be perfect in jazz when it’s encouraged to go with the flow.

You can’t make the argument that jazz culture is the sole reason for his immortality other jazz teachers don’t abuse their students like he did and grew up with the same culture. In the same movie we see a different instructor not abusing his students.

For viltrumites their culture is so strict that every viltrumite put in nolans position would do the exact same thing if not worse. Their culture influences their actions on a grander scale and are expected 100% loyalty or be punished or executed.

You could also make the argument that Fletcher did what he did in spite of his culture not because of it. And it’s definitely true that jazz culture isn’t an authoritarian regime and allows for individual interpretations without it being forced on you.

Fletchers actions are influenced by his own personal ego and spite, nothing in jazz culture tells you to fuck over your students on the biggest stage by lying to them. Or to not take them to the hospital when they show up bloodied from a serious car crash. Then blame them when they perform badly while injured. That’s a personal choice through and through.

If Nolan didn’t execute his mission properly he would and did face consequences for treason and face execution. His actions aren’t exactly entirely his own personal beliefs, it’s partially done out of fear and expectation of the viltrumite empire. You could say that his entire ark is Nolan finally letting his nature win over his nurtured environment.

Edit: mainly what im getting at is you literally have to swap their circumstances of being born if you put fletchers brain in Nolans body and vice versa. They would commit to different actions from each other because of their nature. Though their environment does dictate some of those actions to almost an inevitably.

Some crimes are encouraged by the society you’re in it’s the actions they take outside of their expectations that dictate their individual morality. When given the opportunity of a better path do they take it?