First I'd like to say that I have my own core beliefs, principles and values. My beliefs, principles and values are independent of any external influences, although they might influence them.
The narcissist will adapt to the environment and change their beliefs, principles and values depending on the person they are dealing with. The narcissist might say that they are against abuse, but if they are around someone who they hate or envy, suddenly abuse becomes acceptable to them. They have no consistent core beliefs, principles or values because they are heavily dependent on what other people think and what is currently acceptable in society.
This is why it is so effortless for the narcissist to enforce their flying monkeys/apaths/enablers to adopt the same beliefs, principles and values which they have. Flying monkeys/apaths/enablers are narcissistic anyway, they don't have minds of their own.
You will find that the narcissists and flying monkeys will do or say something and consider it acceptable, but if you do or say the same thing suddenly it is no longer acceptable. The rules which apply to you do not apply to the narcissist or flying monkeys. e.g. It is acceptable for the narcissist to verbally, emotionally, psychologically or physically abuse you. But it is not acceptable for you to do this to the narcissist. Narcissists and flying monkeys are hypocrites.
The narcissist has a sense of superiority and entitlement. If they want something, they should have it. If you want something, it isn't considered a priority or something that needs to be fulfilled. The narcissists believes that they are entitled and more deserving than you are. Even if the narcissist's desire is at the expense of your physical health, their desire is still considered to be more significant and of more importance. e.g. You have a physical injury and require treatment. Instead of investing in your physical health, the narcissist will go and splash out on a new car. Their material desires are of more significance and importance than anything you desire or need, even your physical health.
The narcissist's desire for dominance, power, control, attention and admiration overrides any empathy or consideration they would otherwise feel for you. The narcissist is self-absorbed or selfish, with an excessive need to obtain, which leads their desires to override any rules.