Harry Potter survived the AK because he was magically protected by the runes under his crib. Lily had spent months engraving them. She also did the three ships in a bottle, that the potters had. Harry was fascinated with them. Lily knew there was something about them. For magic was a little bit off around them. One was the Bismarck Battleship, one was the Yamato Battleship, the other was a partially destroyed futuristic ship with no name. Lily was intrigued with it also.
She kept putting magic into the partially destroyed or unfinished futuristic ship with no name. Trying to decide how to repair it. Harry kept accidently putting magic into the entire bottle. It was strange. His magic made all three seem brighter, shinier, or even newer. His magic is why she thought the ship might be unfinished, instead of partially destroyed. The more she could see, the more it looked uncomplete. She read a number of science fiction stories when she was younger. Her magic and mind used those thoughts to help repair or complete the third ship.
The night of Samhain or Halloween as it is called by many, is a night the barriers between worlds thins. This is the reason that Darkness likes to attack, as they get a bit of a boost in power. In this instant, Lord Voldemort is wrong. Many of his victims cry out for revenge or justice. Harry had been making his plushie's move and getting the bottle, but nothing else. It was the reason James did nothing to bind it. He knew once the two's hit, they might indeed need to do so.
Snape had gotten a promise from the Dark Lord to not kill Lily, Peter had got a promise to let him have sex with her. Bellatrix had asked and received a promise to allow her to torture Lily. She had thought of Marrying James when they were younger. "It's him Lily. Take Harry and flee!" She grabs Harry, not realizing he is holding the bottle of ships. Harry was pulling in the magic that was being used against the wards, but once Lily picked up Harry, power went into her instead of back into the wards.
The AK that killed James went a bit into Harry and then Lily. It caused her heart to ache. Harry luved his mum. Not only as a mum, but as a woman. He was not sure of the feelings about Lily, but much went into the bottle. She activated the Runes under the carpet and crib. The glow of the bottle startled her a bit. She put them both in the crib and finished the activation. Then put a blanket over the bottle, while Harry had one foot on it. It was like they were charmed together.
She readied herself for the ancient sacrificial curse. Death was not happy. He would not get Tom or all of Lily's soul. He could see that Magic had powered the bottle to obscene amounts. He did not notice Fate and Destiny high fiving each other. Magic stood there while the ancient runes, not used in centuries activated with the door being blown in. Harry's forehead was hurt.
Tom figured they had a magic dampener in the house. He would look for it when he was done. Three times she asked him to take her instead of Harry. the larger part was leaning toward Death. Magic stared at the forty-two percent that a gold knot and line went to the ship in the bottle. She looked and saw Fate and Destiny doing a jig behind Death. She nodded to them, then turned to watch Tom betray the magic of promises and vows. Tom tells her about Severus. Her heart completely breaks.
She knew Peter betrayed them, when she tried to use the broom to fly out the window and it was a dud. So she was inflamed with vengeance. Finding out he promised Bellatrix she could torture her, was the part that sent her love and magic at Harry. For she knew she was to die now. The AK finished the splitting.