Disclaimer: Harry Potter belongs to J.K. Rowling. I make no claims to ownership.

How Severus Snape lost Lily Evans: A composition dissecting lines from DH.

"You are," said Snape to Lily. "You are a witch. I've been watching you for a while. But there's nothing wrong with that. My mum's one, and I'm a wizard."

Petunia's laugh was like cold water.

"Wizard!" she shrieked, her courage returned now that she had recovered from the shock of his unexpected appearance. "I know who you are. You're that Snape boy! They live down Spinner's End by the river," she told Lily, and it was evident from her tone that she considered the address a poor recommendation. "Why have you been spying on us?"

"Haven't been spying," said Snape, hot and uncomfortable and dirty-haired in the bright sunlight. "Wouldn't spy on you, anyway," he added spitefully, "you're a Muggle."

Though Petunia evidently did not understand the word, she could hardly mistake the tone.

"Lily, come on, we're leaving!" she said shrilly. Lily obeyed her sister at once, glaring at Snape as she left.

It's fairly obvious that he was spying, and Lily would have been frightened and uncomfortable with that. Also, it is shown here, in a strange way, that Petunia was actually concerned about Lily. Otherwise she would have just left her behind.

"It is real, isn't it? It's not a joke? Petunia says you're lying to me. Petunia says there isn't a Hogwarts. It is real, isn't it?"

"It's real for us," said Snape. "Not for her. But we'll get the letter, you and me."

…

"You've got loads of magic," said Snape. "I saw that. All the time I was watching you..."

Here, we see Snape driving a divide between Petunia and Lily. Though he doesn't care about Lily being Muggle-born (probably because he is eager to have a friend, something I can actually begrudge), his dislike for Muggles is another thing that's obvious. And the creepy stalking… Do I actually need to go into that?

"That's where you're going," said Petunia with relish. "A special school for freaks. You and that Snape boy...weirdos, that's what you two are. It's good you're being separated from normal people. It's for our safety."

Lily glanced toward her parents, who were looking around the platform with an air of wholehearted enjoyment, drinking in the scene. Then she looked back at her sister, and her voice was low and fierce.

"You didn't think it was such a freak's school when you wrote to the headmaster and begged him to take you."

Petunia turned scarlet.

"Beg? I didn't beg!"

"I saw his reply. It was very kind."

"You shouldn't have read – " whispered Petunia, "that was my private – how could you – ?"

Lily gave herself away by half-glancing toward where Snape stood nearby. Petunia gasped.

"That boy found it! You and that boy have been sneaking in my room!"

"No – not sneaking – " Now Lily was on the defensive. "Severus saw the envelope, and he couldn't believe a Muggle could have contacted Hogwarts, that's all! He says there must be wizards working undercover in the postal service who take care of – "

"Apparently wizards poke their noses in everywhere!" said Petunia, now as pale as she had been flushed. "Freak!" she spat at her sister, and she flounced off to where her parents stood...

Siblings say a lot of hurtful things to each other when they're fighting, especially when one is jealous of the other. However, this also implies that Lily would never have looked at Dumbledore's letter without Snape's influence.

At last he stopped, outside a compartment in which a group of rowdy boys were talking. Hunched in a corner seat beside the window was Lily, her face pressed against the windowpane.

Snape slid open the compartment door and sat down opposite Lily. She glanced at him and then looked back out of the window. She had been crying.

"I don't want to talk to you," she said in a constricted voice.

"Why not?"

"Tuney h-hates me. Because we saw that letter from Dumbledore."

"So what?"

She threw him a look of deep dislike.

"So she's my sister!"

"She's only a – " He caught himself quickly; Lily, too busy trying to wipe her eyes without being noticed, did not hear him.

"But we're going!" he said, unable to suppress the exhilaration in his voice. "This is it! We're off to Hogwarts!"

She nodded, mopping her eyes, but in spite of herself, she half smiled.

"You'd better be in Slytherin," said Snape, encouraged that she had brightened a little.

"Slytherin?"

One of the boys sharing the compartment, who had shown no interest at all in Lily or Snape until that point, looked around 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.

"Who wants to be in Slytherin? I think I'd leave, wouldn't you?" James asked the boy lounging on the seats opposite him, and with a jolt, Harry realized that it was Sirius.

Sirius did not smile.

"My whole family have been in Slytherin," he said.

"Blimey," said James, "and I thought you seemed all right!"

Sirius grinned.

"Maybe I'll break the tradition. Where are you heading, if you've got the choice?"

James lifted an invisible sword.

"'Gryffindor, where dwell the brave at heart!' Like my dad."

Snape made a small, disparaging noise. James turned on him.

"Got a problem with that?"

"No," said Snape, though his slight sneer said otherwise. "If you'd rather be brawny than brainy – "

"Where're you hoping to go, seeing as you're neither?" interjected Sirius.

James roared with laughter. Lily sat up, rather flushed, and looked from James to Sirius in dislike.

"Come on, Severus, let's find another compartment."

"Oooooo..."

James and Sirius imitated her lofty voice; James tried to trip Snape as he passed.

"See ya, Snivellus!" a voice called, as the compartment door slammed...

What James and Sirius did was wrong. I'm not actually trying to defend it. However, I believe Slytherin had developed a bad reputation by this time, so I can understand where James was coming from. Snape antagonizing him wouldn't help matters. Also, it is apparent that Lily had no problem sharing a compartment with them until Snape came. And Snape should have known that Lily being in Slytherin, which hates 'Mudbloods,' would have been the worst thing for her.

"...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 Avery and Mulciber! Mulciber! What do you see in him, Sev, he's creepy! D'you know what he tried to do to Mary 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 – "

"What about the stuff Potter and his mates get up to?" demanded Snape. His color rose again as he said it, unable, it seemed, to hold in his resentment.

"What's Potter got to do with anything?" said Lily.

"They sneak out at night. There's something weird about that Lupin. Where does he keep going?"

"He's ill," said Lily. "They say he's ill – "

"Every month at the full moon?" said Snape.

"I know your theory," said Lily, and she sounded cold. "Why are you so obsessed with them anyway? Why do you care what they're doing at night?"

"I'm just trying to show you they're not as wonderful as everyone seems to think they are."

The intensity of his gaze made her blush.

"They don't use Dark Magic, though." She dropped her voice. "And you're being really ungrateful. I heard what happened the other night. You went sneaking down that tunnel by the Whomping Willow, and James Potter saved you from whatever's down there – "

Snape's whole face contorted and he spluttered, "Saved? Saved? You think he was playing the hero? He was saving his neck and his friends' too! You're not going to – I won't let you – "

"Let me? Let me?"

Lily's bright green eyes were slits. Snape backtracked at once.

"I didn't m ean – I just don't want to see you made a fool of – He fancies you, James Potter fancies you!" The words seemed wrenched from him against his will. "And he's not...everyone thinks...big Quidditch hero – " Snape's bitterness and dislike were rendering him incoherent, and Lily's eyebrows were traveling farther and farther up her forehead.

"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."

Harry doubted that Snape had even heard her strictures on Mulciber and Avery. The moment she had insulted James Potter, his whole body had relaxed, and as they walked away there was a new spring in Snape's step...

Let's start with how he's pretty much guilt-tripping her into staying friends. That's what, "We're best friends, right?" is a classic example of. Then he's trying to drive the rod even further between Lily and Marauders. He hangs out with a bad crowd, and usually, that says something about your own character. Lily is obviously beginning to realize this by the pensieve scene.

"I only came out because Mary told me you were threatening to sleep here."

"I was. I would have done. I never meant to call you Mudblood, it just – "

"Slipped out?" There was no pity in Lily's voice. "It's too late. I've made excuses for you for years. None of my friends can understand why I even talk to you. You and your precious little Death Eater friends – you see, you don't even deny it! 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?"

He opened his mouth, but closed it without speaking.

"I can't pretend anymore. You've chosen your way, I've chosen mine."

"No – listen, I didn't mean – "

" – to call me Mudblood? But you call everyone of my birth Mudblood, Severus. Why should I be any different?"

He struggled on the verge of speech, but with a contemptuous look she turned and climbed back through the portrait hole...

And Lily says it all right here.

Furthermore:

Yes, James was the one who attacked Snape first, simply because Sirius was bored, but he didn't do anything harmful. Snape cursed him and inflicted actual pain. Mortifying Snape in front of the student body is horrible, but this is Severus Snape, who invented the spell Sectumsempra. A student who embarrasses another might get in trouble, but a student who hurts another definitely gets in trouble. Humiliation might lead to suicide, but wounds can lead to immediate death. See the difference?

I'm basing this only on the books, not information that came out afterward because let's face it, James was further vilified by Rowling to make Snape look good and Severus seem like a reasonable name for Harry's son. I will never, never support a Nazi who let children be tortured in his concentration camp. His work for Dumbledore was heroic, but would you name your son after a member of the SS?

(For example, as of 2009, Reiter's Syndrome is now known as Inflammatory Arthritis. Hans Reiter was a doctor who experimented on people in concentration camps.)

Like a Nazi, Snape chose to join the Death Eaters. He wasn't threatened with death if he refused, and even if he were, he could have said yes and then gone on to save people, but he didn't. So long as Lily wasn't hurt, he was okay with everybody else dying. This isn't love; it's selfishness. That makes him a disgusting human being, much worse than a bully.

Also (and this might be because we only have Harry's POV) he sucked as a spy. He gave very little information to the Order, and more in return to Voldemort. The only time he really protects Harry is in first year, and possibly fifth year when he refused to give Veritaserum to Umbridge.