I have an alternative explanation for Harry's Dark Side: Harry's mother is narcissistic, impressed by education, and not particularly smart.

"by far the simplest explanation for this unverbalizable fear of yours is just the fear of losing your fantasy of greatness, of disappointing the people who believe in you" (ch. 77) is textbook thinking for a child of a narcissistic parent. The child feels perpetually ignored because the narcissistic parent needs validation from the child's accomplishments but refuses to actually listen to the child. Thus, the child is spurred to ever greater heights of intellectual achievement by the parent's need for more status and withholding of love.

"The black rage began to drain away, as it dawned on him that...his family wasn't in danger" (ch. 5) suggests that Harry's Dark Side is Harry desperately trying to stay close to his family.

Typically, children of narcissistic parents inherit either narcissistic or people-pleasing traits. Comparing Harry’s personality to these traits (google “Children of narcissistic parents”, 1st link) shows that Harry has textbook narcissist traits:

Grandiose sense of self-importance? Check. He wants to “optimize” the entire Universe

Obsessed with himself? Check. He appears to only care about people who are smarter or more powerful than him -- people who can help him. He also has contempt for most students and their interests (Quidditch, etc.)

Goals are selfish? Check. Harry claims to want to save everyone, but in practice he tries to increase his own power most quickly. The saving-Hermione thing is still selfish because Harry sees Hermione in the same way he sees his mother -- weak in many ways and bound by emotions and convention, but someone Harry must impress. “It’s disrespectful to her, to think someone could only like her in that way” (ch. 91) makes sense in that a child would be disgusted by Oedipal implications. If Harry’s mother was not narcissistic, then Harry would not have worked so hard to impress Hermione and would have been less disgusted by the thought of being sexually attracted to her.

Troubles with normal relationships? Check. Harry is playing high-stakes mind games with those he is closest to (Quirrell, Draco, Hermione, Dumbeldore), which is emphatically not normal friend behavior. Harry has contempt for nearly everyone else, and is currently hiding alone under an invisibility cloak.

Becomes furious if criticized? Check. When Snape mocked Harry in Potions class, Harry tried to destroy Snape’s career. Quirrell explained, “When it looked like you might lose, you unsheathed your claws, heedless of the danger. You escalated, and then you escalated again.” (Ch. 19)

Has fantasies of unbound success, power, intelligence, etc.? Check. Harry wants to conquer the entire Universe with the power of his intelligence.

Believes that he is special and should only be around other high-status people? Check. Harry avoids average students when possible, and certainly does not hang out with them for fun. A possible exception is Harry’s army, but minimal text is devoted to Harry instructing them, while much text explains how powerful (in battle) and high-status the students in the army have become. In Harry’s mind, the army is a tool to use and an opportunity to show off, not an opportunity to give back and help friends improve their skills for their own sake.

Requires extreme admiration for everything? Check. Harry takes anything less than admiration for his brilliance as an insult, and responds by striving for new levels of intellectual achievement and arrogance, until the others recognize his dominance. Quirrell’s lesson on how to lose described how to avoid making powerful enemies, not how to empathize and care for others -- the insatiable need for admiration is merely delayed and repressed, not corrected.

Feels entitled - has unreasonable expectations of special treatment? Check. Harry requires subservience from the school administration, and special magic items such as the time-turner. “McGonagall said, "but I do have a very special something else to give you. I see that I have greatly wronged you in my thoughts, Mr. Potter...this is an item which is ordinarily lent only to children who have already shown themselves to be highly responsible” (Ch. 14).

Takes advantage of others to further his own need? Check. "I only used you in ways that made you stronger. That's what it means to be used by a friend." (Ch. 97)

Does not recognize the feelings of others? Check. “Er, can I take it from this that you have been through puberty?" (Ch. 87)

Envious or believes they are envied? Check. Quirrell said to Harry, “You have everything now that I wanted then. All that I know of human nature says that I should hate you. And yet I do not. It is a very strange thing.” (Ch. 74)

Behaves arrogantly? Check. “Minerva's body swayed with the force of that blow, with the sheer raw lese majeste. Even Severus looked shocked.” (Ch. 19) I can’t think offhand of a single instance when Harry is not arrogant.

"The Drama of the Gifted Child" by Dr. Alice Miller (Google for the .pdf) spells out what a child raised by one narcissistic parent and one distant parent typically looks like. It looks a lot like H.J.P.E.V.

I can relate personally -- my mother drove me to extreme academic success so she could feel good about herself. I succeeded, then (99th percentile on standardized tests, attended a prestigous and excellent college that I will not name here) but it harmed my ability to relate to people, to distinguish my actual dreams from my mother’s grandiosity/insecurity, and to succeed academically once I had surpassed my mother’s ability to judge progress. When I first found HPMOR I was overjoyed that I was not alone in my arrogance and hope for immortality. It’s only recently that I realized why that is, and what that cost. “Every time I call on it... it uses up my childhood.” (Ch. 91). Suffice it to say that I studied obsessively for years.

I thought about spoiler-warning this to avoid memetic hazard, but apparently on this site “that which can be destroyed by the truth should be”.