Author's Note: Alright, so I have a couple one shots that include Snape as a ghost. While this was, at first, my way of sticking to cannon while also having Snape in the Next Gen universe it has become a matter of minor debate. As such I decided to write another one shot, really quickly, about Snape's death and how, exactly, he became a ghost.
Snape's Death
Snape looked into the concerned green eyes, Harry's eyes, Lily's eyes, until the darkness consumed them. A few moments later he realized his eyes were closed. He opened them. He found himself in a huge expanse of white. He noticed he was naked and clothes appeared at his feat. He put them on and looked around as the whiteness began to ebb away, but before the scenery could form a silver doe came walking towards him, his patronus. As the doe approach it began to color and turned into a real doe and before it reached him it turned into Lilly.
"Lilly?" he asked breathlessly, what had she done to deserve hell? That was where he was sure he was headed.
"Hello Severus," she greeted, smiling at him, like she had in the old days.
"Where are we?" he asked.
"I don't know, it's different for everyone," she answered, she looked around for a moment, "it looks like a forest path."
Snape looked around. The whiteness was fading into a forest path. It was familiar to him. He'd visited this forest many times in his life. In fact, he was at a fork in the path.
"What are you doing here?"
She laughed that beautiful laugh of hers, "I've come to great you!"
"Why?"
"I've missed you, but there's something wrong," she said, wrinkling her brow in confusion.
"What's wrong?" he asked, his heart nearly stopping, which was odd, how can a dead man's heart stop?
"I can barely see you," she answered with concern, then she reached out a hand and it passed through Snape's arm, "I can't even touch you."
Snape looked down at himself in horror. Where Lilly was bright and radiant he was hidden in a black cloud. It was as though his aura or some other non-physical aspect of himself was dirty. He examined himself and found that the cloud was darkest over the place on his arm where he knew the Dark Mark was branded. How appropriate.
"Do you know why I am like this?" asked Snape, hoping Lilly had an answer.
"I have no idea," she answered, shaking her head, then she brightened, "but I know someone who might know."
"Who?"
"Wait right here, I'll go get him," she answered happily, turning back into a doe and bounding away.
When she returned she was a doe and had a large black dog by her side. Snape recognized Sirius' animagus immediately. He withdrew a step automatically, as both doe and dog transformed into Lilly and Sirius.
"Hello Severus," greeted Sirius with a broad smile.
Snape just looked at him angrily. What was he doing with Lilly? Of course, he was in Hell, "Is James here too?"
"Well of course he is, he's dead also," answered Sirius rolling his eyes, although there was no animosity in his tone.
Snape just glared at Sirius, the dark cloud around him darkened to the point that when he looked down he could only see the faint outline of his own hand.
"Severus," sighed Sirius, "I'm trying to help you. I had the same problem you're having. When I first got here I was surrounded by a dark cloud also. It's negative emotions, anger, hate, guilt. Those are things that belong among the living, not the dead. You can't truly join the dead until you give those emotions up."
"I do not wish to discuss this with you," sneered Snape.
"Oh Sev, please, let him help," begged Lilly, pleading with those beautiful green eyes of hers, "just give up the bad feelings."
He looked at her and wanted to cry. He wanted to do anything she asked of him. He tried. He could forget the anger and hate, but the guilt. How could he forget the guilt? How could he ever forget that because of him his beloved Lilly had been murdered? How could he forgive himself for allowing her to be ripped from the earth? He'd known Voldemort was looking for her, he should have killed him, he could have, but he didn't.
"Lilly," he choked, "I can't."
"Oh Sev," she murmured, trying to cup his face in her hand, but her hand just passed through, "you must. If you don't you will be like a ghost among the dead."
"I don't know how."
"There is a way to learn," murmured Sirius from where he stood, behind Lilly, he retreated to allow them a private moment.
Snape looked up, wary, but trying to be cordial in response to Sirius' apparent change to a friendly demeanor. The dark cloud lightened marginally.
"You could return to the world of the living, as a ghost," explained Sirius carefully, "While on earth you can learn how to let go of these emotions. When you are finally free of them you automatically return to the dead."
"How do you know this?" asked Snape, still suspicious.
"Regulus had the same problem as well. Most people have the problem to some extent, but Regulus was particularly bad. He was given the option of becoming a ghost as well, but was able to shake the darkness before going to that extreme," answered Sirius nervously.
"It is impossible," answered Snape, referring to the idea of letting go of his guilt.
"Maybe you should go back to the living. It might take a while, but when you're ready you'll be able to be truly dead."
"Sev," sighed Lilly, "I want you with me," Snape felt a stabbing pain of guilt for causing the death of such an innocent creature, "but Sirius is right. If you need to return to earth to make peace with yourself, maybe you should."
"I want to be with you," answered Snape, looking deep into those beautiful green eyes, "but I can't forget."
"Yes, you can," she answered firmly, and the strength of her conviction, her belief in him, gave him the hope that maybe, he could, "You must believe that you can. Tell me you believe you can."
"Well," murmured Snape, "I suppose someday I might be able to."
"That's the spirit," she grinned, Snape couldn't help but smile back wanly, "we'll wait for."
"It might take a long time," answered Severus, looking nervous.
"There is no time here," she answered with a laugh, "It will be no trouble at all to wait. Besides, I want to wait for you."
"We want to wait," added Sirius with a teasing grin.
Snape refrained from glaring, if he was going to forget his anger, hate, and guilt, he might as well get over the easier two, anger and hate, now. A fair portion of the dark cloud lifted.
"If you wish to return to the living you must go back the way you came," explained Sirius, point down the path as Snape's back, "when you come back we'll all go down that way," he pointed down one of the forks in the path.
"What's that way?" asked Snape, pointing down the only other path.
"That's for those who have no hope for being recovered, it is a bad place of suffering, but we won't let you go that way," answered Lilly, smiling at her oldest friend, "you're a good person."
Snape raised an uncertain eyebrow at her, but she just smiled, and in the end he smiled back. He looked down the path back to the living. It seemed dark. He took a breath and started down the path. He watched as the scenery went pure white again and continued walking. Soon everything went black again and he realized his eyes were closed. He opened them and found himself hovering above his own, still bleeding body as the students rushed out the door. He took a deep breath. Somehow he had to figure out how to let go of his guilt. The task didn't seem easy or enjoyable, but he knew that he would do it. He had nothing but time, and an eternity with Lilly waiting for him in the end. He smiled slightly. He was starting to feel less burdened already.