Despite his attraction to the Dark Arts and Voldemort's ideology of wizard supremacy, Snape's love for Muggle-born Lily Evans, Harry's mother, eventually compelled him to defect from the Death Eaters. He then became a double agent for Albus Dumbledore and the Order of the Phoenix.
Why did Snape tell Voldemort about the Seven Potters?
Even You-Know-Who can't split himself into seven. So, Potter — some of your hair, if you please." In order to preserve his position as a trusted Death Eater, Severus Snape was forced to tell Voldemort the true date at which the Order planned to move Harry, under the orders of Albus Dumbledore's portrait.
Snape's obsession with the Dark Arts and lack of emotional support eventually led to him joining Voldemort's ranks. Voldemort ultimately killed Lily, and Snape appeared to stay loyal to his cause. However, this event led to him working as a spy for Dumbledore.
Used it to sever the hand of a fellow Death Eater, who had been aiming at Remus Lupin's back during the Battle of the Seven Potters, but due to a slip, he accidentally cut off George's left ear instead, leaving him guilt-ridden.
His negative experiences with his muggle father and neglectful witch mother contributed to his decision to join the Death Eaters. Snape's strained relationship with Lily, who chose James and allowed him and his clique to bully Snape, further pushed him towards the Death Eaters.
Why Snape Never Actually Died - Harry Potter Theory
Why did Snape switch sides?
Despite his attraction to the Dark Arts and Voldemort's ideology of wizard supremacy, Snape's love for Muggle-born Lily Evans, Harry's mother, eventually compelled him to defect from the Death Eaters. He then became a double agent for Albus Dumbledore and the Order of the Phoenix.
Shortly before Lily Evans was murdered by Lord Voldemort, Snape changed sides and became a member of the Order of the Phoenix as well as a double agent during the Second Wizarding War. With tremendous difficulty, Snape prevented Lord Voldemort from learning the truth about his loyalties.
Vulnera Sanentur was the song-like incantation of a healing spell which was used to heal deep wounds, which could also be used as the counter-curse for the curse Sectumsempra.
It's implied that Septum Sempra is Snape's own creation, that he used a specific counter-curse to heal Draco, and that the spell isn't designed to be instantly fatal, but to inflict a gruesome, but relatively slow death if left untreated. It's likely Snape is the only person who knows said counter-curse.
The professor drew his wand, and muttered an incantation that "sounded almost like a song." He performed this counter-curse three times and with that, Draco was lifted into a standing position. Snape informed him he still needed to go to the hospital, and ordered Harry to stay behind.
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.
Did Snape know Harry was under the invisibility cloak?
Dumbledore was doing James a favour by helping him pass down the Invisibility Cloak to his son Harry. Snape definitely knew because in book 4 when Harry sees crouch in Snapes office, he's in his invisibility cloak. As soon as snape sees the golden egg and the map he says 'potters here' to mad eye. Potters here.
His father was neglectful and sometimes abusive, which may have contributed to Snape's disdain for Muggles. At some point during his school years, he decided to reject his father's name entirely, giving himself the moniker "The Half-Blood Prince" with his mother's maiden name instead.
Harry describes Umbridge to Sirius Black, saying that she's vile enough to be a Death Eater, despite not being one. Umbridge is also depicted to be a prejudiced person, despising half-breeds and Muggle-born witches and wizards to the point of firing Hagrid due to his half-giant birth status.
Why did Snape betray the Order during the battle of the seven Potters? - Quora. So although I don't ever really defend Snape, in this instance… Snape was instructed by Dumbledore to give Voldemort the true departure plan for Harry - except for omitting that there would be seven Harry Potters.
Dumbledore was dying of a curse set by Voldemort, and only had a few weeks to live. If he died of the curse, that meant Voldemort had killed him and would get the mastery of the Elder Wand. If he killed himself, there was a possibility the Wand would still interpret that as Voldemort's victory.
Ultimately, it's a good thing that Dumbledore's plans didn't work out because if Draco hadn't disarmed the headmaster, Harry wouldn't have gained the Elder Wand's power when he later disarmed Draco. This was proof that, unlike Dumbledore, Harry was destined to reunite the Hallows and become Master of Death all along.
In Book 7, he chose to not rat out Harry and his friends' true identities at Malfoy Manor. He didn't want to be a Death Eater, once he realized what being a Death Eater truly meant. He didn't want to be on Voldemort's side, once he came to truly understand what Voldemort stood for.
When Harry critically injured Draco Malfoy with the Sectumsempra curse, it was actually enough to have him expelled, as Professor Minerva McGonagall stated Harry was lucky not to have been, due to Snape giving lenience to prevent others from finding out that he was the one who invented the curse.
There is likely a good reason that the Sectumsempra is not an unforgivable curse. It's mostly because Sectumsempra is Snape's personal curse that he created. It is not a widely known or used spell, and it is one that he kept secret — which means it can't be added to the list since no one really knows about it.
What was Harry's punishment for using Sectumsempra?
Snape saved Draco, and having realised that Harry got a hold of the old textbook, he punished Harry with a multitude of detentions for nearly killing Malfoy. Harry, despite disliking Malfoy, did not truly want to harm Malfoy to such an extent, and was both horrified and guilt-ridden by using the curse against him.
When Snape heard the prophecy and pleaded with Voldemort to spare Lily's life, he may have inadvertently caused his downfall. While we know Voldemort killed Lily, he did give her a choice. If she stepped aside and allowed him to murder Harry, he would spare her. Lily refused and sacrificed herself to save her son.
McGonagall later learned that Snape was secretly Harry's protector and had killed Dumbledore at Dumbledore's request. McGonagall then made a controversial decision to add Snape's portrait in the Headmaster's Office as a testament to his bravery.
On turning eleven, Umbridge started attending Hogwarts School of Witchcraft and Wizardry. She was Sorted into Slytherin and her head of house was Horace Slughorn.
