Is Their "Ex" The Narcissist, Or Are They?

One way to tell if a person might actually the Narcissist, or if they're telling the truth about being the "victim" of their past relationships with alleged abusers or crazies:

How are their exes doing (or ex-friends or family members)?
Do their exes seem like they're recovering from some kind of traumatic time in their life, or are they doing pretty well, even quite well?

How many of their exes are doing well, compared to how many are not, who appear to be having a hard time of it?

Narcissists use the resources of whoever they attach themselves to, or whoever they happen to be attached to because of family or work or community relationships.
They don't support another person who is in any kind of need (unless they're pretending to in order to gain their trust and get closer to them.)
They don't support the other person, even though the person is supportive toward them, so the other person often ends up becoming very depleted in most or all ways (emotionally, spiritually, financially, mentally, etc.)
They often destroy or take away the means of self-support a person had.
They also take away the person's support network if they can (friends and family), if the people around them are easily influenced. Many people are, and will turn on a person quite easily if they're led in that direction.
Because they are typically abusive in many ways, they often leave deep scars on a person who they were connected to in relationship, and the closer the relationship (the more the person trusted them), the deeper the impact.

So the target of a Narcissist often ends up in a much worse position than they were before the met them, and can end up with a lack of "friends" who were apparently superficial in the first place, or they would still be around, especially when the person went through such trauma. 

Since Narcissists only take but don't give, they often end up in better positions, and often continue to gain and gain throughout their lives. They also tend to be good at cultivating and maintaining superficial "friendships" that don't require any genuine, real-life-mutual support like one would need to give in a healthy friendship, family or romantic relationship, so they often appear to be surrounded by good relationships.

Narcissist Formula

Don't forget the formula:
The more you are dealing with, the less the Narcissist can "tolerate" being supportive of you. The more knowledgeable, skilled, or talented you are, the more they will try to put you down, invalidate you, belittle you, or sabotage you. The more impassioned you are, the more "annoyed" they will be with you. The more you talk, the more they feel like they're not getting to talk. The more you help them, the more they resent you. The kinder and more supportive you are toward them, the LESS they will be toward you.
And don't forget about the stereotypes, gender roles, and bigotry (sexism and racism, heightism, ageism, classism, etc etc).
It's all real, and it's all the time.
THEY are not YOU, THEY don't control LIFE on this planet, or YOUR life.

If They Want You To Be "Wrong"

If a person wants you to be wrong, resents or envies you, or wants to have control, favor, or popularity over you for whatever personal issues they have, they are likely to:

argue with you about anything

"snap at" you about anything

criticize you for anything at all

judge you as inferior

be purposely non-supportive of your endeavors or goals

be purposely non-supportive of you when you have difficulties

put you down, insult you, make fun of you, harass you

imply that you are crazy, self-centered, or histrionic

make demands of you

not use "manners" or courtesy when talking or interacting with you

cast blame and shame on you for random things

be dishonest with you

invalidate whatever you say or express

say that you're wrong no matter what you're saying (even if you repeat what they just said a few minutes ago, verbatim)

treat you like you don't belong

try to make you feel like you don't belong or "unliked" by treating others around you with attention and courtesy, but NOT you, on purpose.

may try to turn others against you

~
These behaviors are flags that you are dealing with a person of lesser maturity who may have a problem with entitlement, a lack of boundary health, and is probably very insecure, or fearful, or intimidated by you. 

Abuse Can Cause Mental Illness

Constantly being exposed to abusive people behaving abusively can and often does cause mental/emotional illness. Especially for targets of abuse, and especially for targets of abuse who are not being supported or protected by those around them.
Typically in communities and families where Narcissism is present, people often abandon or attack a target of abuse INSTEAD OF supporting or protecting them.

Mental/emotional illness from the trauma of abuse and abandonment can be healed, however it may require completely removing one's self from one's original environment, creating distance or blocking contact completely from those who were abusive and also from those who were purposely non-supportive.

Controller Narcissists Isolate Their Targets

One of the main reasons Controllers and Narcissists isolate their Targets from others is so that the Target will not have anyone who is supportive of them or protective of them.

A Narcissist will do all kinds of things to isolate a Target from others, and then claim to be a person who cares and is supportive of them. But when the Target seeks even minimal support about anything, the Narcissist reacts with venom, criticism, aggression, or dismissiveness.

They don't simply want some control over the Target, they want complete control. They don't want the Target to have ANY support from any other source.

They don't want the Target to receive any neutral or positive feedback from others either, because neutral and positive feedback from other people is one of the main ways human beings build and maintain their confidence and self-esteem, and know that their mental and emotional health is sound.

Stop The Blame Game, It DOES Take A Village To Raise A Child (Properly)

Only a complete fool puts the blame solely on one (or both) parents when their son or daughter has apparent behavioral or emotional problems in adulthood, including anxiety or depression.

The larger part of a person's life path and behavior was developed because of their experiences during childhood. Children are directly influenced by the rest of the family members, by the kids and adults in the community they grow up in, by the way they're treated by the various people around them, and by the way each of their parents are treated by other people.
They're also influenced by media, but that's only in relation to the influence of the real people around them.

If identical twins were born with identical genetic mental and emotional health and intellect, but they were raised in two distinctly different families and communities (for example one in a supportive atmosphere with minimal sexism, racism, and denial, and one in a chaotic, abusive, or neglectful one), they might retain certain similarities with one another but for the most part they would be very, very different from one another as adults in values, outlook, emotional health, behavior, and life choices.