Voldemort told Snape he had been a faithful servant, but that he had to regrettably kill him, as he mistakenly believed he was the master of the Elder Wand, as Snape had killed Albus Dumbledore, its former owner the year prior, during the Battle of the Astronomy Tower.
Voldemort thought Snape was the owner of the Elder Wand. Using Avada Kedavra while the Elder Wand was resisting Voldemort was already a risky business, but using it against the true owner would be suicide (as Voldemort would find out when he used the Elder Wand against its true owner: Harry).
"I regret it," said Voldemort coldly. When Voldemort finally works out the truth about the Elder Wand (albeit incorrectly) he sends Lucius to fetch Snape and then confronts him before killing him off. He confesses his apparent “regret” for having to kill him, twice.
Erroneously believing Snape is the master of the Elder Wand and that Snape's death will make him the master of the Wand, Voldemort kills Snape by having his pet snake Nagini bite him through the neck.
He believed if he had used the Avada Kedavara curse, the wand would have resisted him doing so as it did when he tried to kill Harry who is the true owner of the Elder Wand. To avoid this, he slit Snape's throat and then let Nagini attack him.
So Snape asked Voldemort to spare Lily. He basically rejected this request out of hand; killing the Potters was too important to him. He agreed to give Lily a chance to save herself, but he certainly didn't promise to save her life.
Harry's scar had hurt during Arrival Feast because of Quirrell's connection to Voldemort; Snape was then talking to Quirrell, allowing Voldemort, from the back of Quirrell's head, to sense Harry's presence.
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.
But, despite his maltreatment of Harry and the complicated killing of headmaster Dumbledore (who knew he was a double agent?), Snape did sacrifice his life for Harry at the Battle of Hogwarts.
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.
Quirrell didn't use the Killing Curse on Harry, because Voldemort said “use the boy” to get the Sorcerer's stone from the Mirror of Erised. If Quirrell had killed Harry right away, he would have lost the chance to get the stone.
During his final confrontation with Voldemort, Harry knew that he (Harry) was the Elder Wand's true master. He again cast Expelliarmus, ejecting the wand from Voldemort's hand into his own as the Killing Curse rebounded off his body, fatally striking Voldemort.
However, Draco was ordered by Voldemort to kill Dumbledore. Lestrange saw this as an opportunity to test Snape's loyalty and while making this vow to always watch over Draco, Lestrange forced Snape to agree to killing Dumbledore himself in case the youngster was unable to do so.
Sectumsempra is a curse invented by Professor Severus Snape, during his childhood, when he was known as "The Half-Blood Prince". He created it with the intention of using it against his enemies, and it soon became one of his specialties.
Severus Snape is one of the most divisive characters in the entire "Harry Potter" series. He was a double agent, so many character details were kept a secret. His mother was a witch and his father was a Muggle, leading to the moniker "The Half-Blood Prince."
The Tales of Beedle the Bard first appeared as a fictional book in J. K. Rowling's 2007 Harry Potter and the Deathly Hallows, the seventh and final novel of the Harry Potter series. The book is bequeathed to Hermione Granger by Albus Dumbledore, former headmaster of Hogwarts School of Witchcraft and Wizardry.
Snape dismissed the curse as bearing no change to her appearance, causing Hermione to flee in tears. We knew Snape wasn't exactly chummy, but this was a real tipping point.
The Killing Curse ('Avada Kedavra')
Obviously there's no coming back from this spell, and according to the Ministry of Magic's classifications (and, again, fake-Moody) this is the worst Unforgivable Curse.
'The Prince's Tale'
Professor Snape is dead on Voldemort's orders, and Harry saw it all. In his dying moments, he told Harry to take his memories and look at him one last time. Voldemort's voice then sounded through the corridors, challenging Harry to meet him in the Forbidden Forest in an hour. This is the end.
In the first movie the the scene of the troll you can see that snape has a bleeding leg, why? Snape was bitten by Fluffy the three headed dog. When Troll was released, he went to make sure that no one was after the stone as everyone was distracted by the troll.
He gave Harry his tears as they contained all his memories so that Harry could see the complete truth from his eyes. Snape knew that he would not be able to tell Harry everything properly due to his current condition. He also knew how important it was for Harry to know the truth behind Dumbledore's death.