Picking up the monk again. I think I've been overlooking an important cultural perspective, that being Wuxia. This is the centuries old tradition of martial arts warriors roaming the breadth and length of China, fighting evil, pursuing romance, overcoming horrible childhood tragedies and following the codes of loyalty, honor and duty.
I don't mean to say that those codes are something that should matter to the players. I have been opposed to "appropriate behaviour" for a given class since it was first put in place for the paladin, thereafter becoming a major set piece of the corporate game forever after. If you want to play a cavalier, a barbarian or whatever, you must behave according to these precepts that absolutely deny you any freedom of will or purpose. It has always been nonsense, no matter how many people like the code. Codes, even in the real world, can always be broken ~ and it never ends in losing our knowledge or our ability.
This is, in fact, a fundamental conflict in Wuxia: that the evil master in the story is just as capable, just as dangerous, as the honorable master. In the stories, for the protagonist to win, there must always be some other thing that compromises the evil master: a minion of the evil master, a helper of the honorable master, circumstance . . . and most commonly someone sacrificing their lives in order to win. Most often, the evil master is killed at the cost of the honorable master, producing a zero-sum gain that is treated as a bittersweet victory. Yes, Li Mu Bai is dead, the romance is shattered, but the evil has been stopped. Everyone else may now live in peace.
I have no interest in restraining any character in my world with such melodrama ~ if they want to pursue it, that's fine, I'll create that adventure and give it the nuance it needs. But that has to be a player choice. Wuxia likes to teach that the ambitious master must always be taught in the end that true satisfaction demands a quest of peace, love, family or the simple life, lived simply. I have zero interest in that for D&D characters. As monks wandering the world, having adventures, they should not be held back, but allowed to act as they will.
It is my role to give them their abilities, not to dictate how those abilities should be used.
This is why I hesitate to pursue things like the monk eschew attachment and gaining pleasure from the real world. That is a trope. That is not the freedom to adventure. The literature is full of proof that monks can be evil, that they can still be tremendously able yet interested in acquisition and power. Just because the stories all insist that these people must die in the end doesn't mean role-play has to work that way.
Players should be free to be as evil ~ or as good ~ as they personally desire, without paying an penalty regarding their ability whatsoever. The danger comes from combat against the other, not the player's agenda against moral-making rule systems.