Some decisions change the course of history.
Lily did not know that she was about to make one such destiny altering decision when she bade goodbye to Severus in such an unscrupulous manner.
It had indeed been with a heavy heart that she had done so. It was almost like she had been looking for a chance to oust him from her life, she was looking for an easy way out perhaps; because she knew, deep inside her, that she had enough reasons to be grateful to Severus and forgive him for the momentary lapse of judgment.
She knew that he had never really meant that insult.
But, the times were tough and called for difficult decisions, even if escapism and cowardliness were a part of those decisions. And at that point, she had definitely needed to make one. She had to pick a side and this had been the easiest way to do it. (And that's how Lily would go on to defend her goodbye to herself.)
Everybody else, of course, was rejoicing Lily's choice. All her friends thought that she had finally come to her senses. James Potter's grin seemed even wider than usual and his bullying had gotten even worse.
Several weeks had passed since Lily's and Severus' unceremonious parting. Although Lily hated to admit it, she was missing her former best friend terribly. She missed their long walks, she missed the way he made her feel better about her misunderstandings with Petunia, she missed his Eureka moments of invention of new spells.
She especially missed those Eureka moments.
"Sev!"
No response.
"Sev! Severus Snape!"
Severus was startled out of his trance by Lily's shrill voice. He looked at her in complete annoyance.
"Yes Lily! Would you please stop shouting and let me get on in peace? Go away somewhere, for heaven's sake!"
"What on earth is going on in this place?!" Lily demanded, completely ignoring his request to leave him alone. Her almond green eyes took in all of the cauldrons strewn around as well as one pewter vessel containing a silently boiling mud-colored liquid.
Severus sighed. She wasn't going to give up so easily.
"I'm trying to make a potion that'll make people's voice go mute for a few days..."
"Oh my God! Isn't that illegal?"
"No, my dear naive Lily! It isn't illegal," said Snape and went back to stirring his potion.
And then suddenly, out of the blue, he shouted, "Lily...Lily run! Go away! Now!"
"Why?! What's wr...," And with that, Lily's voice was gone. No sound came out of her mouth. Severus' potion had worked its magic! She stood there, simply mouthing words like an utter idiot. She stared at her friend in horror, her mouth going awry as though she was trying hard to vomit something.
"Oh, wow! It worked! It worked! It worked, Lily!" shouted a thoroughly exhilarated Snape.
Lily's eyes narrowed in what was unmistakable anger. But it only seemed to further excite Severus.
Instead of helping her regain her speech, he remarked, "You look so cute when you're all riled up, you know!"
Lily gave Snape an icy cold glare and turned and walked away.
She had eventually regained her voice after Snape had forcibly administered her an antidote later that day. And despite her annoyance with him, she did see the power of that potion: any person who naively inhaled the potion without taking the antidote prior would not be able to make any sound at all. This was a superb defence against most of the witches and wizards who were unskilled at nonverbal spells. It would effectively deprive them of their spell power!
That had been one of the several moments of sheer brilliance displayed by Snape. He was probably the cleverest student among many generations at Hogwarts, she had to hand that to him.
She was extremely tempted to repair the friendship with him, to accept his apology. It took her a great deal of self-control (and a few self-control spells on herself) to not go back to him.
Lily's world had shrunken significantly after two of her most valuable associates, Petunia and Severus, had exited it. While she did have good friends, none of them came close to filling the void that Severus had left. For the first time in six years, Lily wanted to escape her beloved castle. She did not want to be a witch anymore. People were disappearing, families were being torn apart, friends were becoming foes. Anarchy was slowly taking over the wizarding world. There was depression everywhere. Nothing seemed to be going right.
Lily was slowly falling apart.
