Constant Thrum Of Tension

That constant, continuous undertone of tension the controller seems to emit may likely be something they developed in childhood. They may have picked it up from someone else in the household, or they may have felt it themselves.

Many children grew up inside of a household where one of the adults was always wondering what another adult was going to do next, or not do next, or what they were doing while they were out. One parent might have been in a continual bad mood because either they have mood or control issues, or because the other parent has mood or control issues, or treated them with disrespect. A parent who is controlling or haughty often emits this tension constantly, and children in the house may try to "match the frequency" so they can feel normal.
Children who grow up in a household where one parent is gone a lot for any reason may develop this; they may be stressed for the absence of the parent, or their other parent may be stressed from it;  and also those who had one parent who was physically ill, or another family member, especially if there was always some sense of foreboding, like something bad could happen at any moment.

Children who grew up in such an environment can adapt this constant thrum of tension and incorporate it into their personality, often feeling like it's the way one is supposed to feel when one is an adult, or when one is "responsible", or "in charge". 
The feeling of "waiting for the other shoe to drop", or being worried that one's daily, normal behavior and actions will be judged and micro-managed, are symptoms of a controlling household. And that feeling can be incorporated into a child's regular every day personality. Without it, the person may feel like he or she is giving up their autonomy and personal power; they may actually believe it's part of being an adult human being. We all incorporate our childhood environment into our selves as we grow up, it's normal; so if this constant tension was present, it can easily be taken in by a person who grew up in it.

Self Confidence vs. Narcissism

Self-confidence is not the same as "Narcissism".
When you are confident, you feel solid and good about yourself and your capability, you aren't plagued with self-doubt, and you don't have the need to compare yourself to others. You are happy about your accomplishments, your abilities, and your aspirations, and are not thinking much about what other people think of you; you're just content and happy about what you're doing,  improving your skills, and learning things. You notice when others are disrespectful toward you, and you automatically stand up for yourself. You also stand up for others. You don't pretend disrespectful behavior is not happening, or allow  others to insult you, control you, put you down, condescend, or walk on you, or do any of that to others around you.

You learn from others regardless of their "station" or "status". Self-confident people ENJOY sharing ideas, listening to others' points of view about all kinds of things; they enjoy discussion, they don't insult others, condescend, try to overtake a discussion or counter. They don't assume negative things about others; they don't participate in gossip. They put their energy into improving their own life, and the lives of those around them, not in tearing others down or picking them apart. They don't waste time "critiquing" others lives or bodies; they don't invite or embrace negativity, judgment, or divisiveness. They notice when others do, and they may avoid entire groups of people who are negative and controlling.

They are HAPPY when they meet a "kindred spirit", someone else who shares their interests, someone else who enjoys similar things or does something that they do. They are happy for those who are doing well and who have accomplished things. They LIKE IT when someone else knows things they don't, understands things they don't, or is better at something than they are, because it's an opportunity to LEARN. They don't gloat, brag, or boast. They enjoy honesty, good manners, ethics, and critical thinking, and they find relief in taking accountability for their mistakes and transgressions. They LIKE to hear what others have to say, and they LIKE to learn from others, and they truly APPRECIATE and acknowledge the talent, skill, ability, and knowledge of others; they don't try to shush them up with insults and countering, invalidate them, dismiss them, or shun them.

A self-confident person does not brag or boast; they could have 3 PhD's and keep it to themselves. They could have climbed Mt. Everest twice and not tell others unless the subject comes up in conversation. They could be a world-reknowned artist and you wouldn't know it if you met them, because they don't seek extra credit and admiration, so they don't talk about it unless someone asks. They don't feel "outshined" easily, and they don't envy those who do something well that they also do. They also don't hesitate to share their accomplishments with others when it occurs to them to do so, and often do when they're happy about something they've done. They don't HIDE what they've accomplished, and at the same time they don't think of their accomplishments, skills, or talent as "bragging rights".

They have no need to PROVE that they are better or more knowledgeable at something that someone else, and will even pretend to know less than they do, or are less capable than they are, if it seems like someone's ego is fragile, or if they are under fire from others.

Those who are self-confident LIKE TO promote OTHERS, they ENJOY pointing out what others are accomplishing, and help them gain credit and acknowledgment for what they do. Even when it's the same thing, the same field, the same skill, the same talent, and even if they are not receiving any benefit themselves for promoting them.

Those who are self-confident have the emotional, mental, and physical well-being of others in mind, regardless of whether they are a "close friend" or not.

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Those who are Narcissistic only have their own well-being in mind, and will emotionally, mentally, or physically neglect, shut out, or abuse others if it serves them in any way. Occasionally they can seem somewhat concerned for others whom they see as also "elite" who they consider to be in their circle of elitism.

Narcissism is not the same as self-confidence, and can be said to be it's opposite. It's all about comparing one's self to others, and trying to find them to be lesser than one's self, OR trying to associate one's self with those who are seen as "higher".
Narcissism craves attention,  admiration, an audience, and compliance, not equable connection.
Narcissism feels uncomfortable when it feels like it "has to" listen to someone ELSE, especially if it wants that person to be their designated audience, caretaker, sidekick, or to be "lower status".
When Narcissism sees someone else doing well, displaying talent, showing skill or knowledge, or looks happy and confident, it feels resentment, envy, intimidation, or a loss of control. It may even feel hatred. Narcissism needs to be the one who is in control, the one who is seen as MORE~ the one who is smartest, the most capable, the most beautiful, or the most talented. Narcissism doesn't do things WITH others, it doesn't care about "kindred spirits"; it doesn't feel inspired that you're a singer too, or an artist too, or that you're also into science, or love animals, or also a writer, or also a parent. Narcissism instead sees you as either a threat to their spotlight, or as some kind of "wanna-be", or even a sycophant. Unless you are someone they see as "above" or "elite", Narcissism does not want to hear your point of view and is not interested in doing anything WITH you. If you are going to be around Narcissism, you are expected to let THEM be the "one in charge", the one who gets the attention from others, the one who is liked and admired MORE THAN you. It can't even be equal, it has to be them getting more attention, admiration, and adulation.
If Narcissism doesn't get to be the star, the boss, the one in control, the one who is MORE SKILLED, more experienced, knowledgeable, capable, or intelligent, the one who is seen as innocent and above reproach, the one who is above the "rules" (too cool for school), the one who is RIGHT and righteous all the time, no matter what, then they can become angry, hostile, and resentful. Sometimes even dangerous, and are not above "retaliation" for their feelings. Narcissism sees all of their feelings as being CAUSED BY other people and things.

Narcissism ONLY feels "good" about their own skills, abilities, or talent if there is someone to put down, someone to tear down, someone to compare themselves to, someone they can control, or someone praising them profusely or showing adulation.

Self-confidence is definitely not the same as Narcissism.