Here's my take on what actually happened in the dojo incident described in chp 19.

Voldemort went there in disguise to learn the valuable martial art. He reacted badly to losing so they put him through the ordeal he described. He went along with it because he wanted to learn the martial art. The ordeal did teach him valuable lessons about losing, and he vowed to learn to control his temper and master tactics of ingratiation and supplication to better manipulate others. But he felt angry and humiliated by it (as he expected Harry to be), and also vowed to return and fulfill his revenge fantasies. So after he mastered the martial art and left the dojo, he came back openly as the Dark Lord and killed them to live out his revenge fantasies and to prevent others from learning the skills (keep science secret). He spared one student who had befriended him (and who probably stood up for him during the ordeal, like Draco to Harry), and he had that student spread the version of the tale that he wanted told (to maximize fear while hiding some of his true powers, and to deflect attention away from the value of that martial art).

Slightly edited the original post to avoid giving away what my readers have finally convinced me is, in fact, an undesirable spoiler. I also hope you didn't mind my removing the mention of FAI, because I feel fairly strongly about not mixing that into the fic. "A fanatic is someone who can't change their mind and won't change the subject"; if we can't shut up about FAI while talking about Harry Potter, we may have a problem.

The fic now has a hate blog dedicated to it.

It seems like the spells in the HP universe are complicated and abstract enough that they must have been designed (programmed?) by wizards long ago, who added them to the laws of the universe and left them there.

Now, if I were designing a spell like the Killing Curse, I would include a little easter egg/safety mechanism: after a thousand castings, it backfires. Choose a number large enough that only a major dark wizard like Voldemort will encounter it, so it doesn't hit some minor villain and spoil the surprise. (Alternatively, rather than counting kills, count evilness, with killing a baby counting for more evilness points than an adult. That would explain why it backfired on Harry Potter, rather than some other victim.)

This is the most sensible explanation I can come up with. Or it could be that it backfired because the third through fifteenth places of the decimal expansion of the local humidity were a prime number, or something similarly arbitrary. But I would be disappointed if it was something like that. (I would also be disappointed if his parents came up with a spell that reflected it, because everyone seems convinced that no such spell is possible.)

That was a hard swat at "The Ones Who Walk Away from Omelas".

As for why Harry has such an exaggerated sense of responsibility, it might be that growing up on science fiction thing. A lot of science fiction is set up so that the hero can have a huge effect in a satisfying way. Perhaps Harry should have balanced it with reading history. On the other hand, he's living in fiction, so maybe he's right for his situation.

Lois McMaster Bujold has described sf as fantasy of political agency [1], and I think she's on to something.

I assume that shutting down Azkaban has a political solution rather than a magical or violent solution. This will be interesting to watch.

Why would Snape ask Harry for his take on Snape's past? One of the underlying premises of the story is that the smarter characters (possibly with the exception of Hermione) always have a deeper plan. Did Snape actually expect to get good advice? To be told that all his choices were correct? To have a reason to be angry at Harry? None of these make huge amounts of sense (to me, at least-- I have trouble keeping track of all the scheming), even though the scene was very emotionally effective.

This is basically my review posted to fanfiction.net-- let me know if there's a problem with reposting such here.

[1] The link goes to quite an interesting speech