Poe establishes at this point how in the viewpoint of an insane individual, the line distinguishing insanity from rational thinking becomes blurry. Indeed, the Mad Man's illusion that he is not insane and was fortunate to experience a "sharpening of the senses" was one way of illustrating this blurring of distinction between illusion and reality, sanity and insanity. The Mad Man did not realize that murdering the old man because of his blue eye is irrational; for him, the threatening feeling that he experiences whenever he sees the blue eye is reason enough for him to kill the eyes' owner, who is the old man. In this example, his illusion is that the blue eye is watching and threatening him, while the reality is that he killed the old man because of this perceived threat.
The emotional instability that the Mad Man experienced became evident as the story draws to a close. Though he tries to demonstrate sanity by providing a detailed and objective narration of the manner in which he killed the old man, the Mad Man betrayed himself as he sought to "eliminate" just the source of his unsettling feeling -- the blue eyes, and not the old man:
It was open --wide, wide open --and I grew furious as I gazed upon it. I saw it with perfect distinctness --all a dull blue, with...
And have I not told you that what you mistake for madness is but over-acuteness of the sense? --now, I say, there came to my ears a low, dull, quick sound, such as a watch makes when enveloped in cotton. I knew that sound well, too. It was the beating of the old man's heart. It increased my fury, as the beating of a drum stimulates the soldier into courage.
This passage revealed the truth about the Mad Man's true condition. He is not only losing control of his own mind and emotions, he is also losing control over the direction of his narration. Gradually, his insanity is laid bare for the readers to witness. Gradually, his projection of a rational-thinking individual is dissolving to give way to a confused person, who wants to take control of his self and environment by eliminating the things and people who he feels are threatening to him. It is through this crucial moment that the Mad Man succumbed to his insanity, and allowed himself to commit murder. Gone was the rational individual, and the unstable, highly-emotional Mad Man finally takes over, revealing his murder and insanity.
