If you read his rather autobiographical poem "Alone," you'll read that he always saw things differently than others did. He was not moved by the same passions as others, nor did he react as others did. Poe's life was a series of tragedies, all adding up to some kind of mental disturbance. We'd be better equipped today to diagnose and treat such things. It's interesting to think of "what might have been" in such a case.
I agree that there is no diagnosis of insanity, but I think a case could be made for bipolar disorder (aka manic depression). It seems that Poe had periods of enormous creativity followed by bottoming lows. He also had a habit of writing about the "double self," thus lending credence to the belief that he felt torn between two existences.
Some of his behavior was quite odd as well. I recall reading in his biography that when his wife (and 13-year-old cousin) died, he had a strict ritual of walking around her grave a certain number of times each night, perhaps indicating OCD (obsessive-compulsive disorder.)
Of course, it's impossible to diagnose after-the-fact, but many depressives tend to self-medicate with alcohol, perhaps temporarily relieving his pain but ultimately exacerbating his condition.
Is Stephen King insane? Or any other author of horror and/or gothic fiction? Despite the dark nature of his poems and short stories, we don't have the evidence to say whether he was mentally ill. What we can say from them is that he certainly knew what madness looked like. Could a person who was truly insane describe its effects so well?
Poe knew sadness and grief his whole life. His mother died when he was only a baby, he frequently argued with his foster father, and his beloved wife died of tuberculosis. One morning he was found unconscious on the streets and died not long after. There is evidence that he was an alcoholic and that his death was caused by his drinking. There is even a story that he was a victim of voter fraud. There was a group of people who would take drunk men to various voting precincts, getting them progressively drunker so that they couldn't say they had already voted, and have them vote for a candidate several times. But we have as much proof of that as we have that Poe was insane.
Visit the links below, especially the Poe Museum, for more information.