I think first we must distinguish the differences between being mentally unstable (ie: some sort of psychological disorder) and being "insane" (ie: you're strange, or do irrational things, what have you).
I'm going to take a (probably) strange stance and argue what I think most others would believe to be untrue; that is, I believe those that are mentally unstable do at various points realize that they have gone off the deep end (excluding the most severe cases), whereas people who are either "strange" or do irrational things, most of the time, do not realize that this is so.
Think about it this way. If you're having mental breakdowns to the point of hearing voices or hallucinating, at some point I would argue that you realize those things should not be happening to you. Unless you are born experiencing these symptoms, would you not recognize that something is out of the ordinary?
There is a fine (and arguably impossible to distinguish) line between understanding that something is not right and acting upon such mental instabilities. I guess the words I am looking for here would be the difference between calm and frenzied. Except in the most extreme cases, I doubt that looking at it retrospectively, mass murderers can say that their actions are justified (ie: rational). Although in the heat of the moment they acted irrationally, they were not using any internal justification for their actions. I am, however, speaking for these people, so this point is probably moot. It is interesting to ponder, however.
Alternatively, if you have grown accustomed to being simply irrational or "strange", how would you know that it is so? I argue that if someone is being irrational objectively, subjectively, they might be making perfect sense. Do you understand what I mean?
I think first we must decide the definition of insane and then see if that definition can be noticed/observed by the insane person.
I tried to get a more detailed explanation than this ^^
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