What's the difference between flaws of character and a personality disorder? Okay, I have read this question and 2 answers and I don't believe these are the same questions.  It's not about legal status or the moral appropriateness of punishment by society.  And it's not about the neural basis of volition (and how that can get messed up, physically), I don't think.

It's more like why isn't some personality disorder (like narcissistic or borderline) simply considered a character flaw?  Like: that narcissist is arrogant and conceited and brags because he wants to be and likes it.  Or: that borderline is demanding and manipulative and hateful because she wants to be and it serves her.  So that is the first question.

The second question is how to relate with these troubled persons? When does one hold the other personally accountable for their decisions and actions and when does one go into treatment mode as a caregiver would a patient or a dependent loved one?  Caregiver is flawed human being with their own issues and not professional in any field related to psychology or mental health.

But, outside the domain of the law and criminal responsibility, how should a regular "civilian" or "layperson" relate to persons (immediate family, extended family, coworker, friend or acquaintance) with NPD or BPD?  When should a person treat the NPD or BPD as a selfish asshole (with moral judgement and sometimes disdain) and when as a sick or weak person that is perhaps just very needy (and/or difficult)?
There is a clear definition of what a personality disorder is. The Wikipedia article on Personality Disorders lists the criteria given in the DSM-5 and ICD-10. Take a look: https://en.wikipedia.org/wiki/Personality_disorder

If you don't understand those definitions, please edit your question accordingly.