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.
Although Snape ultimately betrays the Death Eaters on the whim of Albus Dumbledore, he nevertheless made the conscious effort to join them all those years earlier.
In a misguided attempt to win back Lily's affections, Snape joined the Death Eaters along with a group of his fellow Slytherins. Severus was made a member of the Slug Club presumably because of his talent in Potion-making.
As one of the core Death Eaters, Snape likely committed a number of crimes including murder, torture and more on behalf of the Dark Lord. However, even if Snape didn't commit any of these acts himself, being associated with the Dark Lord would be enough to land him numerous years in prison.
It's never said she knew that he joined. In her rant to him in fifth year she claimed that's where he was headed, but the only proof she had was he hung around some kids who were pureblood elite that didn't like Muggleborns and wanted to follow Voldemort. So at best she made an educated guess.
Snape's love for Lily fed his hatred for James, and meant he would stop at nothing ' even arranging the murder of her child ' to possess Lily. Lily's life would be spared in return for Snape giving information of the prophesy to Lord Voldemort.
He doesn't trust Snape as Dumbledore does, but he believes that he can detect any disloyalty. Moreover, Voldemort regards Snape as highly intelligent and therefore ' as a true Slytherin ' a person whose top priority is to take care of himself. To his twisted mind, selfless equals stupid.
He would always resent Harry for being James' son instead of his. Snape even said that he didn't want anyone to find out that he was protecting Lily's child, “especially Potter's son” (pg. 679 of Harry Potter and the Deathly Hallows) because of his hatred for James.
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.
After eating a foul tasting dragon tartare, McLaggen vomited on Severus Snape's shoes, earning himself detention for the next month. When Ron later asked Harry about the pair, he was "slightly more cheerful" to learn that the date did not go well.
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.
There is no evidence to show that Voldemort ever knew Snape had betrayed him by revealing his plans to Dumbledore; on the contrary, there is a fair amount of circumstantial evidence to indicate that he didn't.
Because Lily (according to them) had the nerve to not fall in love with her former childhood friend, Severus Snape and marry that no-good-evil-spoiled-rich-brat James Potter.
But Snape didn't have time to get too deeply into that. Later in the year, Dumbledore revealed that Harry was in fact a Horcrux, meaning that he would eventually have to die. Now, as firmly established, Snape was not the greatest fan of Harry, but that didn't mean that he ever stopped loving Lily.
Snape's secrecy is revealed to have been a mask and he has sacrificed being well-liked to be able to save Harry, all because of his love for Harry's dead mother: the only friend Snape ever had.
Voldemort recruited him and likes him because Snape had the same kind of feelings toward his Muggle father that Voldemort had toward his own. Snape's father was not a nice man. He was often abusive towards Snape's mother which is why he took the pseudonym “Half Blood Prince” to begin with.
Later in the year, McGonagall duelled with Death Eater Alecto Carrow during the Battle in the Astronomy Tower and later learned of Snape's murder of Dumbledore. McGonagall was initially appointed as Headmistress, but was demoted after Voldemort took control of the school and placed Snape as Headmaster.
Draco's refusal to reveal Harry's identity to Bellatrix was not because he liked him. It was because he believed that Harry was the only chance they had at defeating Voldemort. At first, Draco admired and revered Voldemort until he threatened his family's safety.
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).
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 "Deathly Hallows," Snape also uses his Patronus to help Harry on his quest to find Voldemort's horcruxes; a Patronus charm can be used to send messages, and Harry seems to subconsciously recognize the doe as an extension of his mother, allowing him to trust it.
Image via Warner Bros. Similarly to Harry, Snape's patronus represents a person. In the memories he gives Harry before his death, Snape shows Dumbledore his patronus, using the form to prove his love of Lily Potter, Harry's mother.
I cannot deny, however, the fact that Severus was loyal to Dumbledore, to some extent. He responded very differently to Dumbledore than he did to others. He wasn't rude, sarcastic, nor bitingly cruel to Dumbledore as he seemed to be to many others.
The Death Eaters and the Dark Lord don't simply cast Avada Kedavra on everyone all the time because it doesn't suit their long-term goal of ruling the world and having a wizard-run society.
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.