Her sacrifice gave Harry the power of love, a protection spell sealed with the blood spilled by her death.
So why does Harry always use Expelliarmus? Well that would be just his natural reaction. From the wiki page harrypotter.wikia.com/wiki/Harry_Potter it says that Expelliarmus is his signature spell and would thus be his natural response to any attacks. Just like if person A trained the hammer fist a lot.
"... have mercy..."
Lily's last words were an ardent request to the Dark Lord to let her little boy live. As the books reveal by and by, the fact that she made the ultimate sacrifice for her son sealed Harry with her love, a form of old magic that proved powerful enough to keep him safe in years to come.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
Draco Malfoy's mother Narcissa was cold, cunning and devoted to the Dark Lord. But she was also a mother, which meant she was willing to risk everything to make sure her son was safe. When Harry survived Voldemort's Killing Curse for the second time, Narcissa pretended he was dead so she could get to Draco.
Though Harry encountered many opportunities that deemed Avada Kedavra necessary, it remains as one of the Unforgivable Curses Harry Potter never cast. For one, he viewed the spell as an immoral practice commonly used by users of the Dark Arts.
Dumbledore's last line is very similar in the final book and movie. He tells Harry in the film, "Of course it's happening inside your head, Harry. Why should that mean it's not real?"
Why have we never considered this before? Both Lily and James Potter were killed by Voldemort aged 21 at Godric's Hollow on 31 October 1981, when Harry was just 1 year old, And the rest of the story, you no doubt know.
Cedric Diggory
"Wands out, d'you reckon?" Before Robert Pattinson was an undead vampire, he was a golden boy wizard killed by a Voldemort henchman. His death proved one thing: that Voldemort wasn't messing around. Photo: Warner Bros.
When Harry wrestles Draco's “everyday” wand out of his hand at the Malfoy's mansion, he conquers Draco, and therefore the Elder Wand — hidden in Dumbledore's tomb at the time — transfers its allegiance to Harry. Rowling said her American editor suggested the moment when Harry conquers Draco should be more dramatic.
But when Ron and Hermione are about to die by Nagini, Ron casts the killing curse as a last resort and it simply rebounds off Nagini as it was nothing.
Alohomora, which is a spell that unlocks objects (and it's also Hermine Granger's signature spell).
He truly believed the Elder Wand had to be passed down through murder, because that is what the legends say. But even if he knew that one only had to be disarmed, Voldemort was still too arrogant to do that. He kills anyone in his way.
A doe. And in his final battle with Lord Voldemort, Harry explained the significance of this to his adversary, and to us: 'Snape's Patronus was a doe,' said Harry, 'the same as my mother's, because he loved her for nearly all of his life, from the time when they were children'.
Now, as firmly established, Snape was not the greatest fan of Harry, but that didn't mean that he ever stopped loving Lily. Dumbledore was surprised that Snape seemed to care for the boy. With a swish of his wand, Snape conjured up a Patronus – Lily's Patronus, a doe. 'Always,' he said.
A Professor at Hogwarts School of Witchcraft and Wizardry, Snape is hostile to Harry due to his resemblance to his father James Potter, who bullied Snape during their time together at Hogwarts. As the series progresses, Snape's character becomes more layered and enigmatic.
In the [Deathly Hallows] book, Snape's dying words to Harry were "Look at me". Right then we did not realize the significance of his words but in the very next chapter when Harry goes through Snape's memories in the Pensieve, he comes to know how much Snape loved Lily.
In the scene Petunia says, “ You didn't just lose a mother that night in Godric's Hollow you know, I lost a sister.” But Petunia saying this, I believe is wishful thinking! She was to hardened to love Harry or exchange good words.
The darkest of the Dark Arts, Harry Potter unforgivable curses are the Cruciatus Curse, the Imperius Curse, and the Killing Curse. Although the curses are highly illegal, Harry using Unforgivable Curses happened, but he uses them once he was thrust into the Second Wizarding War.
In the Goblet of Fire book, the Avada Kedavra is described as unblockable. However, in the Deathly Hallows movie, Hermione blocks an Avada Kedavra curse aimed at her.
In order to conjure the avada kedavra curse, you have to want to kill your victim. We all know that Voldemort could easily kill a child without an ounce of remorse... but not Snape. Snape didn't want to kill Dumbledore, and this was why the spell was blue instead of the usual green.