SPOILER ALERT, PART 3: As the release of Harry Potter 7 approaches (July 21), I’ve been theorizing about the biggest mystery of the series: How did Harry become “The Boy Who Lived,” not just surviving a killing curse but somehow causing it to rebound against its perpetrator, Lord Voldemort, destroying his body and exiling what was left of his soul? (See “A Bloody Good Theory” to follow the earlier line of thought.)

We know:
Harry was saved after his mom, Lily, sacrificed her life.
She was extremely good at potions.
A wizard can stash part of his soul in an object or living thing, and said repository is called a horcrux.
To create a horcrux, we know little more than this: Killing is needed, because it rips apart the soul.
Voldemort learned about horcruxes from Professor Slughorn, who taught potions.
Lily Potter was one of his best students, and so was Severus Snape.
Snape, the “Half-Blood Prince,” excelled at inventing potions and jinxes.
Snape may have been smitten with Lily, who took pity on Snape when he was mistreated by James Potter, Harry’s eventual father.
Snape revealed part of a prophecy to Voldemort, which led to the Dark’s Lord attempt on Harry’s life.
Snape regretted this action, and before the attack, told superwizard Albus Dumbledore.
The attack on Harry left him with a lightning bolt scar on his forehead.
That scar hurt when Harry was near Voldemort or when he had visions of horrors being committed by Voldemort and his snake Nagini.
Harry had other psychic connections to Voldemort, as evidenced by his ability to speak Parseltongue (snake language) and by the Sorting Hat’s considering Harry for a house at Hogwart’s school named for a Voldemort ancestor.

See where this is going?

How about this scenario?
1. Snape and Slughorn, guilt-stricken over putting Lily in jeopardy, worked with her to devise a desperate last-resort way to protect Harry. (Clearly, this backs the idea seemingly-sinister Snape is really a double agent, who kills Dumbedore because it’s the only way to keep deceiving Voldemort.)
2. They devise this wild idea: Implanting a horcrux in Harry would render him immune to a Killing Curse. Perhaps it’s simple logic: You can’t use that curse on yourself. It’s like the curse pulls the plug on its source. Or: With a piece of Voldemort in Harry, part of the Dark Lord’s desire to kill the person in front of him was mirrored in Harry, creating a kind of echo curse aimed right back at himself.
3. But how to create the horcrux? Killing and incantations can’t be enough. Look at the all the signs that potions are involved. Lily, Snape, Slughorn — all potions masters. This suggests creating a horcrux involves a potion, and they figured out how it was done, so Lily could use it to protect Harry.
4. For example, say before or after the required murder, a potion is needed to mark the object or creature that will become the horcrux. If Lily marked Harry’s forehead before the attack, then by giving her life, the act split Voldemort’s soul and that streak of portion became the scar and horcrux, protecting him from the Killing Curse.
5. This fits the mother’s love theory Dumbedore expounds, but adds a magical wrinkle as well as the final exonerating evidence about Snape.
6. Likely, this potion involves blood. Look at all the mentions of “blood” in the book, from half-bloods to pure bloods, to blood used in potions and elixirs, like the unicorn blood that Voldemort drank to stay alive, like the potion that used Harry’s blood to restore the Dark Lord’s body. We also know that Snape created a potion that worked in Lupin’s blood to control his werewolf symptoms.  
7. But whose blood? If the victim’s, the blood wouldn’t come from the attack, since the Killing Curse doesn’t leave a mark (except on Harry). So maybe Lily used her own blood in advance. Or, if Voldemort’s blood was needed, perhaps she and husband James obtained some in the wild magical battle that destroyed the house the day they died. Or perhaps Snape somehow obtained some for them. Sounds like a winner to me: Snape gave them Voldemort’s blood, she mixed it with some of her own, rubbed a streak on Harry’s forehead, and, wham!, when she sacrifices her life, he gets magical protection.
8. Oh, Voldemort seemed to want to spare her life. Could this have been as a favor to the Snape?

How Harry saves himself
“Either must die at the hand of the other for neither can live while the other survives,” the prophecy says. Weird. This seems backwards. Could it be read this way: “Neither can live if one dies”? After all, doesn’t “the other survives” mean someone is dead? Look further at the Harry as horcrux idea. It’s been said (Dumbledore and Slughorn, I believe) that Voldemort can only be killed after the horcruxes are destroyed. Does that mean Harry has to kill himself to destroy his horcrux scar? Conversely, if Voldemort kills Harry, would that destroy his horcrux, and make him vulnerable? Sounds like a sticky situation (assuming it’s not as simple as forehead surgery).

Again, I’m figuring there’s some potion solution (pun intended). Remember in Chamber of Secrets how Harry destroys the horcrux that was Tom Riddle’s diary? He uses a basilisk tooth. A snake’s tooth. (Interesting. Harry and LV speak parseltongue … Voldemort has a snake, Nagini, which might be a horcrux, too.) So venom’s part of the potion. What else? How about some blood? Well, what do you know: Before destroying the diary, the basilisk bit Harry. There was his blood on that tooth. Blood and snake venom. How about this, then? Nagini’s fangs figure into destroying Harry’s horcrux. Maybe Harry deliberately lets the snake bite him, then uses the bloody fangs to destroy the scar. I’m guessing it looks like Harry dies, but he’s saved by an antivenom concocted using Gopalott’s Third Law (that antidotes for blended poisons have an extra ingredient besides those that counteract the parts, as explained in Chapter 18 of Half-Blood Prince) and extra ingredient comes from the Felix Felicis lucky potion, or bezoars, which seem to counteract all sorts of poisons, or even phoenix tears again, which saved Harry after his basilisk bite.

UPDATE 6/2: Of course, somehow love must also figure in. So expect Ginny Weasley to strongly figure in. She seems to Harry’s true love, and not coincidentally, she was possessed herself by a Voldemort horcrux, the enchanted diary of his youthful Tom Riddle self (Chamber of Secrets). As a jazzy song says in Ch. 16 of HBP:

“Oh, come and stir my cauldron,
And if you do it right,
I’ll boil you up some hot strong love …”

Funny how that song has echoes of horcruxes, with talk of lovers stealing each other’s hearts:

“Oh, my poor heart, where has it gone?
It’s left me for a spell …
… and now you’ve torn it quite apart
I’ll thank you to give back my heart!”