It's well known even in the first book that Snape is a cold hearted person who has very little sympathy for anyone, even his fellow Slytherins. We find out in the last book that he is still in love with Lily.
My question is this; was Snape's unrequited love for Lily (and therefore the emotional pain that brought) his whole reason for being cold-hearted or was there something else that caused him to be cold?
I know that we don't have a lot of backstory on Snape, but still ...
Probably not.
In the first summer he met with Lily, Snape used magic to send a branch down onto Petunia's head, hurting her. He clearly had a bit of a mean streak even then.
When Harry accidentally uses Legilimency on Snape, he sees a memory of a young Snape in his room as his parents loudly argue outside. Later in the Pensieve memories, Lily asks Snape if his parents are still arguing. He obviously had a very unhappy family life.
By the time Lily breaks off her friendship with Snape in the fifth year, she notes that he is already hanging out with future Death Eaters and throwing around the term "Mudblood" despite his love for her.
Snape was a young man who grew up in a troubled household and fell in with a bad crowd. It'd be more accurate to say his cold-hearted and callous nature caused his love to be unrequited, rather than the other way round.
The earliest age we see Snape, when he first meets Lily, he’s clearly not in a loving home with happy parents. Lily seemed to be what Snape considered the one good thing in his otherwise unpleasant life.
He doesn’t seem to be well-cared for, any time his appearance is described it’s clear he’s somewhat neglected.
“Two girls were swinging backwards and forwards, and a skinny boy was watching them from behind a clump of bushes. His black hair was overlong and his clothes were so mismatched that it looked deliberate: too-short jeans, a shabby, overlarge coat that might have belonged to a grown man, an odd smock-like shirt. Harry moved closer to the boy. Snape looked no more than nine or ten years old, sallow, small, stringy. There was undisguised greed in his thin face as he watched the younger of the two girls swinging higher and higher than her sister.”
- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince’s Tale)
His parents don’t have a pleasant relationship, and argue often. He’s clearly looking forward to going to Hogwarts simply so he could get away from his parents.
“How are things at your house?’ Lily asked.
A little crease appeared between his eyes. ‘Fine,’ he said.
‘They’re not arguing any more?’ ‘Oh, yes, they’re arguing,’ said Snape. He picked up a fistful of leaves and began tearing them apart, apparently unaware of what he was doing. ‘But it won’t be that long and I’ll be gone.’
‘Doesn’t your dad like magic?’
‘He doesn’t like anything, much,’ said Snape.”
- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince’s Tale)
Even Harry notices that young Snape doesn’t look cared for, in contrast to Harry’s father James.
“One of the boys sharing the compartment, who had shown no interest at all in Lily or Snape until that point, looked round at the word, and Harry, whose attention had been focused entirely on the two beside the window, saw his father: slight, black-haired like Snape, but with that indefinable air of having been well cared for, even adored, that Snape so conspicuously lacked.”
- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince’s Tale)
I’d argue it’s the opposite- Lily is the only reason Snape has any shred of good in him.
We see that Snape grew up in a difficult environment. In one of the Occlumency classes with Snape in The Order of the Phoenix, Harry breaks back into Snape’s memories and witnesses a scene of Snape as a small boy, cowering as his father shouts at his mother. That scene fits in with a conversation between a young Lily and Snape in the Prince’s Tale chapter of The Deathly Hallows (as noted in another answer):
“They’re not arguing anymore?”
“Oh, yes they’re arguing,” said Snape. He picked up a fistful of leaves and began tearing them apart, apparently unaware of what he was doing. "But it won’t be that long and I’ll be gone."
"Doesn’t your dad like magic?"
"He doesn’t like anything, much," said Snape.
On top of his home life, Snape integrated himself into a group of nasty Slytherians at school. In the chapter "Padfoot Returns" in The Goblet of Fire, Sirius mentions that “Snape’s always been interested in the Dark Arts, he was famous for it at school.” This fits with the series of Lily/Snape memories from "The Prince’s Tale," starting with an argument that precedes the "Mudblood" incident:
“… thought we were supposed to be friends?” Snape was saying. “Best friends?”
“We are, Sev, but I don’t like some of the people you’re hanging round with! I’m sorry, but I detest Every and Mulciber! Mulciber! What do you see in him, Sev, he’s creepy! D’you know what he tried to do to Marry Macdonald the other day?” Lily had reached a pillar and leaned against it, looking up into the thin, sallow face.
“That was nothing,” said Snape. “It was a laugh, that’s all—”
“It was Dark Magic, and if you think that’s funny—”
[jumping ahead]
“I know James Potter’s an arrogant toerag,” she said, cutting across Snape. “I don’t need you to tell me that. But Mulciber’s and Avery’s idea of humor is just evil. Evil, Sev. I don’t understand how you can be friends with them.”
This scene is a setup to this conversation, which follows the "Mudblood" incident and is the end of Snape’s direct interaction with Lily, as far as we see in the books.
“You don’t even deny that’s what you’re all aiming to be! You can’t wait to join You-Know-Who, can you?”
This response is getting long, but Snape is a character who had a difficult childhood, then found schoolmates who were similarly disposed toward evil acts. He goes on to become a Death Eater, and there’s nothing in the books to indicate he has any second thoughts about this decision up until he relays the prophecy to Voldemort. At this point, his love for Lily, which is the only good thing in him, is enough for him to set aside his life as a Death Eater to become a double agent.
As Dumbledore says in the memory that reveals Snape’s true allegiance in "The Prince’s Tale":
“My word, Severus, that I shall never reveal the best of you?" Dumbledore sighed, looking down into Snape’s ferocious, anguished face. "If you insist …”
> ".
Commented
Dec 1, 2017 at 17:11
I can't give the technical answers like the previous folks, but I think there is a significant point that is missed, but still supports their answers.
We are told there are two things that make a person: nurture (how they are raised) and nature (dna, neurophysiology). There is a third: the choices you make also define the person you become.
Snape grew up in a bad place, but that alone doesn't make him good or bad. It can go either way. Michael Oher from "The Blind Side" is both a character and a real person that grew up in very rough places but the darkness of his childhood was a defining contrast to the light of his person.
Snape had natural propensities. Personally, I suspect he was mildly, and high-functioning autistic. He had his emotions and thoughts that were intrinsic to his person. That alone doesn't make him good or evil. It can go either way. In Lord of the Rings, the ruling ring called Strider by his 'true name' Ellesar, but still his nobleness of character caused him to reject it.
Snape made choices, like hanging out with death-eaters and buying into their bullying and classist/racist ideology. Perhaps it was hunger and desperation much like the starving Germans who bought into Hitler because he fed them. Over time those choices opened some doors and closed others. Those opened and closed doors defined the trajectory of his soul, his inner self. They broke his heart and robbed him of certain types greatness.
While it is speculative, there is a very good chance that he saw his mother physically and emotionally abused as a child, swore he would never be his father - taking an oath against his own soul - and when he saw what had happened to Lily in Godrick's hollow, realized to his greatest despair that he was worse for Lily than his father had ever been to his mother. It was likely more devastating than anyone who has been through such a background, and had to look in the mirror at the monster is capable of seeing. His most virulent violence was most likely reserved for himself, poised against the inner Snape, and the smallest incidentals of it were what the students experienced.
Personally, I think the Lilly thing was the main source, but not the only one. For example, when the branch hit petunia, it was because he momentarily lost control because he was a child and had little control of his magic, and Petunia was insulting them both. The other reason he was spiteful was because life had put him through a lot of terrible stuff and that will eventually turn anyone bad, especially if you never had an escape. Years of suppressing spite later, after the death of Lilly and all the other stuff, ( plus working at Hogwarts only because he had no other way of protecting Harry ) to guard a boy who looked like James Potter and BOOM, spiteful. Don’t forget that the students weren’t very nice, just because he was strict and not well suited to the job. A few years of that will make you hate the job completely. Although, he did sometimes do good for reasons besides Lilly, like stopping Quirrell.
Snape is a twisted, evil and bigoted lunatic. The fact that he loved Lily is the only positive in his entire existence. His brilliancy, skills would have made him another Dark Lord if Lily had never happened to him.
The only 3 people other than Lily that Snape ever treated well were Dumbledore [sycophantic and survival reasons], Malfoy - Draco and Lucious [Sycophantic and survival reasons still] and Tom Riddle Jnr [Sycophantic and survival reasons]
So it is obvious Severus Snape is incapable of being good to anyone, and the only goodness in him happened because he met Lily.
