In the last decades, the traditional assumption that the family is primarily responsible for the development of personality disorders has been challenged. For example, although it may be tempting to assume that aggressive and abusive behaviour in adult victims of child abuse is caused by these traumatic experiences in childhood, the evidence is correlational at best, which does not rule out other explanations such as genetic factors or gene-environment interaction. There is now considerable evidence that part of the personality disorder is related to genetic factors. Of course, providing a safe structured family environment may reduce the risk that children eventually will get a personality disorder.

A well-informed psychtherapist will recognize a personality disorder.

That is a difficult question which is not easy to answer, given that there are a number of different personality disorders officially acknowledged as such. It is difficult to find an open access publication in which these various personality disorders are described. You might find some of this information in Wikipedia: https://en.wikipedia.org/wiki/Personality_disorder

Good question which is difficult to answer, because not all persons with BPD are comparable. If the person acknowledges him/herself that he/she has been diagnosed with BPD, you can even discuss your worries with him/her in the context of the PD and say that you understand that this statement of yours may hurt him/her, which you understand, but that it is better for your relationship is etc...

Usually this starts gradually in adolescence. Whether mental health care is needed depends on the suffering of the individual and his/her environment.

That is indeed difficult. Perhaps it helps if you realise that it is an illness. If you continue to feel guilty you may consider going to therapy yourself to learn how to cope with this difficult situation.

Not every vulnerable narcist wants you to be angry or abusive towards him/her.

There are now a number of evidence-based treatments for a number of personality disorders. Results of these therapies result in less complaints, but generally do not change the personality fully.

A central characteristic of avoidant PD is social avoidance, together with hypersensitivity to negative evaluation, fears of rejection, and feelings of inferiority. There is some research to suggest that temperamental vulnerability might increase the risk for avoidant PD. Children who are behaviourally inhibited, shy, or hypersensitive to harmful stimuli might be vulnerable to develop avoidant PD. A number of studies have evaluated the effects of psychological treatments with patients with avoidant PD as the primary complaint. The results of these studies show that various behavioural strategies (exposure and social skills training) are moderately effective (for review see https://www.routledge.com/Personality-Disorders/Emmelkamp-Meyerbroker/p/book/9781138483057

A number of studies have revealed that a substantial proportion of wife batterers are characterized by Cluster C personality disorders. In incarcerated spousal killers, murdering one’s wife is better predicted by avoidant personality traits than by psychopathic traits. Other research showed that obsessive-compulsive PD was the most prevalent personality disorder in men convicted for severe intimate partner violence (see Emmelkamp & Meyerbröker, Personality Disorders, 2020). https://www.routledge.com/Personality-Disorders/Emmelkamp-Meyerbroker/p/book/9781138483057

Unfortunately, there are no evidence-based interventions available for individuals with antisocial PD. Generally, clinicians are highly pessimistic about the results of psychological treatment of antisocial PD and psychopathy. Although this pessimism may be understandable for the treatment of true “psychopaths”, a negative attitude with respect to the possible outcome of psychological treatment of antisocial PD may not be totally justified. As with borderline PD, there is evidence that some individuals with antisocial PD may “mature out” of the disorder over a long time period, perhaps due to reduced levels of impulsiveness. Factors associated with good outcomes in individuals with antisocial PD are job stability, no alcohol abuse, and a stable marriage.