How would you describe yourself? Do you perhaps think you're insignificant, just a speck of ephemeral dust in the universe, carried about by the wind? Perhaps you believe you're the smartest man in the world, endowed with incredible mental skills that justify tormenting dependants? Both of these signify mental issues, but for now let's focus on the latter, which is roughly the description of megalomania.
According to current psychological theory, there's no cure for megalomania, only treatment for its various stages. The core cause of megalomania is an amalgamation of an inferiority complex and a detachment from reality that feeds on itself and inflicted trauma. A megalomaniac has never had a wakeup call, meaning he essentially lives in sleep, loves it and does not intend to wake up. The two main stages of megalomania are hyperactivity, during which the person is unstoppably active, and depression, dead stillness of body, heart and mind. Someone suffering from megalomania will typically have decades where hyperactivity or depression stages dominate, normally in youth and old age respectively. Each stage is treated with a separate medication: sedatives for hyperactivity and bringer-uppers for depression.
Having had personal experience with what to me appear to have been megalomaniacs, I believe I can provide some insight into what the warning signs are. A megalomaniac will pick on weak persons, claim absurd things that put him in the position of power with regards to the weak person and then violently defend his side. This invokes conflict and generally produces trauma that megalomania feeds on. For example, a megalomaniac might state, "I'm always right" and then proceed to engage in trivial arguments with anyone nearby who might be a suitable target. Contradicting a megalomaniac in this state is guaranteed to lead to a physical, violent struggle. All of this is perfectly intentional but may appear as if though it's happening at random. There's no remorse, regret or any kind of contrition on the side of megalomaniac that it came to blows over trifles; if anything, the aggrieved person has to apologize to retain any semblance of a relationship, which further fuels megalomania.
Paranoia, unfounded fear for one's safety that borders on hysteria, is another warning sign that trouble is afoot. The megalomaniac can't accept that things simply happen on their own, such as that wind can blow and knock over a vase; it was an intentional act by a traitor, prompting cries in the vein of, "Why is everyone trying to ruin my perfect life?" This line of thinking serves as the perfect excuse why the megalomaniac has trouble succeeding and also doubles as another way to pummel the self-confidence of dependants to feed his megalomania. Insomnia is common as well, with any sudden moves or sounds jolting the megalomaniac fully awake, who is perfectly convinced everyone's plotting to get him, further feeding paranoia, conflict and delusions. There is simply no possible way for any number of dependants to cut through this labyrinth of mental illness in that environment; any breakthrough can only occur through outside causes (illness, physical disability or imprisonment).
End result of megalomania is total rot: mental, emotional and physical. Unable to perceive any change in his environment (not a positive one, at least), megalomaniac will keep reinforcing the same thought patterns he's had over decades. Simply put, any attempts at a discussion result in talking to a broken record that keeps repeating the exact same words and phrases, including the very same intonation and inflection. Emotionally, the megalomaniac can't feel anything new or even deign to have eye contact, as if there's a barrier that prevents any connection or shared experience. Finally, the depression stage usually involves sitting or laying down for most of the day, causing sores and various health problems related to atrophy of muscles and poor circulation. Megalomaniac ends up a total wreck of a person that is carefully avoided by non-dependants.
Trying to reason with a megalomaniac is a futile, maddening affair that should be avoided at all costs. There's simply nothing to be said to such a person to bring him in touch with reality; he will just keep doing what he considers right, no matter how much it hurts others. It goes without saying that a megalomaniac will cheat in any way possible to get his way, such as by lying, twisting words, changing the topic, mean accusations and outright denying even the most indisputable facts, even when there's no need for it. Why? Because he's always right and in his mind reality has to bend to meet his will. For a mental illness, megalomania is terrifyingly consistent and impervious to any logic. If caught in a lie, megalomaniac will simply start throwing punches.
A megalomaniac will typically end up isolated and completely deprived of normal human contact due to relentless imposition of his will, resistance to that from non-dependants and inability to take any criticism from non-dependants, withdrawing from the outside world. This leads to lashing out at dependants, who become emotionally crippled and have to take the full brunt of the assault. Eventually the megalomaniac is left all alone when everyone realizes there's no person inside the meat shell, only mental illness, and leaves. Even the closest allies and family members have to develop bitterness, lose all hope and eventually give up when they get whipped by the megalomaniac in this manner over the course of years and decades. It can seem heart-rending but it's perfectly voluntary and planned for; in the end, megalomaniac gets the perfect environment where he's always right because there's nobody left to dispute his words. The sooner you notice the signs, the better for your own sake.
Dealing with a megalomaniac from the dependant position is like dealing with a kid that's holding a gun – you have to appease the little brat, chipping away at your psychological integrity to avoid bodily harm. Don't argue with the megalomaniac. Call the police as soon as you notice the argument escalate verbally; it's a prelude to assault. Once you've got the police in, simply explain what happened and preferably get some evidence of the threats, such as audio recordings. After you've reported the problem to the police a few times, you should consider fighting back and using whatever physical force is necessary to subdue the megalomaniac, and then again call the police to report it. Whenever possible, make it public who started the argument and that you were simply defending yourself.
Take care of your bodily safety, do not engage in arguments and do not tolerate any physical violence or even unwanted contact, such as poking, grabbing hair, body-checking, scratching or slapping. All of these constitute physical assault. It's crucial that you always respond to force immediately in such a way that ultimately subdues the maniac or the cycle of abuse will absolutely intensify through repetition. Megalomania does not subside, it only gets more and more elaborate, perhaps by involving your other family members and loved ones to humiliate and hurt you even more. If you do nothing, your spouse and kids will absolutely get abused by the megalomaniac in perverse ways. Do not lose your calm and never apologize to the megalomaniac, because it simply fuels the delusion. Understand that megalomania is a mental illness that completely consumes a person. There's nothing left beneath the surface of the meat shell, it's all gone.
Eventually, you'll have to put physical distance between you two, preferably by moving out. Plan for it and be patient. The goal is to become self-sustainable and in such a well-defended position that the megalomaniac has to fight an uphill battle according to his own rules. For example, a megalomaniac might say, "My house, my rules" to justify abuse (which does not hold up legally, by the way), in which case you moving out and the megalomaniac coming to visit you implies you're in charge now. All of a sudden, you can say to the megalomaniac to shut the fuck up and behave with respect or you'll punch him in the face and kick him out, the same way he threatened or perhaps did to you. The look of shock and total collapse of delusion is perhaps worth all the years of abuse and might even be the only way to bring a megalomaniac to his senses.
So, how could you tell if you're going to develop megalomania? It's easy, just notice your reaction when receiving criticism. If you can tolerate criticism and constructively criticize the critic in response, you'll develop a high tolerance for mean words and also get a wealth of input while handling hecklers. For example, I was at one point approached by a middle-aged man who tried to AMOG (alpha man of the group) me by saying I have an ugly ass because my genes are bad. There must have been a pun in there somewhere though our exchange wasn't in English but I took it in stride and said, "Well, it's not like I can control my genes. If you could tell me a quality that I can actually work on, that would be something." To that the critic had no response except to bow his head and start twiddling with his phone.
Realize that most critics are unfair and see only the bad sides. This isn't a reflection of the world but a reflection of their inner state. Focus on positives and try to suggest a course of action rather than just saying "this bad". When presenting a person's point of view back to it, do your best to articulate it as clearly as possible, which will lead to people happily presenting their half-baked thoughts to you; they'll understand you won't throw it back in their face. If you disagree with something, do preface it with "I feel" or "I think" but don't try to present it as an universal fact. Be kind towards others because that's the best way to eventually be kind to yourself.