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.
Snape's redemption came when Lily was murdered by Voldemort. His resentment towards James and others disappeared as he realized the cruelty of the Dark Lord. Dumbledore trusted Snape and allowed him to avenge Lily's death, even though he didn't fully forgive him.
He "went back to" being a death eater when Voldy returned to power even though he was not devoted. He was really giving info to the order. He was fascinated in Dark Arts and he wants to join him but after he found out Voldemort killed Lily he switched hus loyalty to Dumbledore and become a double-agent.
The final book, Deathly Hallows, explains this. Snape HAD BEEN a Death Eater. He later renounced his connections with that group, but no one besides Dumbledore knew for sure whether he had truly switched sides, or was just faking it. Dumbledore, alone, knew the truth.
Snape seems to shift his loyalty between Dumbledore and Voldemort throughout the Harry Potter movies. Only the last film reveals that Snape had always been on the good side. He swore to protect Harry and did Dumbledore's bidding in his war against Voldemort, including infiltrating the Death Eaters.
Snape's childhood wound, emotional neglect, loss of his best friend, and unrequited love for Lily compounded, and he succumbed to the dark side in Harry Potter. He found himself spying for Lord Voldemort, but all this came to a screeching halt when the Dark Lord marked the Potters for death.
Snape was born to Eileen Prince, a witch, and Tobias Snape, a Muggle, making him a half-blood (hence the name, "Half-Blood Prince"). This is rare for a Death Eater, as remarked in the last book, though Voldemort himself also had a Muggle father.
Summary. As a character, Severus Snape was often misunderstood. Many believed he was a villain, and weren't convinced he was actually working for Dumbledore. Snape killed Dumbledore in an effort to protect Draco Malfo and spare him from the emotional trauma of murder.
Snape Would Never Have Gained The Elder Wand's Loyalty (Even Without Draco) Dumbledore knew that he couldn't allow the loyalty of the Elder Wand to fall to Voldemort, so he developed a plan. He told Snape that he must be killed at his hand before the curse took his life.
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.
He joined Dumbledore to ensure Lily's protection, and after her death, her son Harry's. Albus Dumbledore's relationship with Severus Snape showcased an unusually strong trust and loyalty.
As much as people romanticize Snape's infatuation with Lily, the fact she did not love him back romantically is ignored. The fact Snape and Lily were only friends, and she only saw it as such, reveals a lot about his refusal to respect her boundaries.
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.
In the heat of the moment, Snape lost control and called Lily the forbidden word. He called her a mud-blood by accident because he was frustrated that lily did not like his closest friends. He did not mean it.
Why did Snape stay loyal to Dumbledore after Lily died?
But when Lily's life is directly threatened, he has to make a choice, and he chooses to stand between Voldemort and Lily. To do that, he aligns himself with Voldemort's chief opposition, Dumbledore, and becomes his spy. Even after her death, he remains at Dumbledore's side at Hogwarts and vows to protect her son.
Why Snape's Avada Kedavra was blue when he killed Dumbledore?
The intent of the caster is a vital component of the spell. While Snape was still able to produce Avada Kedavra without any intent to kill, being a great wizard, the mix of hesitation and necessity probably resulted in the spell being blue.
If Draco hadn't disarmed him, Snape could have “stolen” the Wand from his body and hidden it somewhere. (Again, this only works if Voldemort is not looking for it.) He could then present it to Harry as a token of loyalty to get Harry to actually listen to him.
Thanks to his own and Severus Snape's skills, Snape was able to hold off the curse from spreading, but it only gave him a year to live. Knowing that Voldemort had given Draco the task to kill him, and knowing that he would fail but not wanting his soul broken if he did, he asked Snape to do it instead.
Severus Snape was born 9 January 1960 to Tobias Snape, an abusive Muggle, and Eileen Prince, a neglectful pure-blood witch. He began to identify with his mother's family and created a secret nickname from his mother's maiden name, calling himself the "Half-Blood Prince".
By Harry Potter and the Deathly Hallows, it's a stretch to call Draco a villain anymore. We just watched — or read — him struggle to reconcile his choices and morals in The Half-Blood Prince. He can no longer justify his actions, and is far more aware of the evil he was born and raised into.
He was eventually forced to accept that his father was the Muggle parent, and that his mother was the magical one. It was around this time that Riddle gave himself the alias "Lord Voldemort", to spare himself of the reminder of his "filthy Muggle father".
The movies follow the books closely enough that they aren't really separate universes, except for the fourth movie. There isn't any truly compelling evidence that Snape is Harry's father, because all the points you raise are explained. Snape hated Harry because Harry reminded him of the man who "stole" Lily.
In the Harry Potter and the Deathly Hallows movie, Snape showed how much he loved Harry, providing him with his memories and showing that he saw Lily in him, his last words echoing this: "You have your mother's eyes."
