Severus Snape is a human being, and no human being is perfect. He has made numerous mistakes in the past. One that he regrets the most took place when he was but a boy that was struggling between his act as a Slytherin, soon-to-be Death Eater and as a childhood friend to Lily Evans, the girl he admires. On a seemingly perfect day, he directly insulted her for helping him. From then on, everything fell out of place.
He joined the ranks of Voldemort. He became one of his most loyal supporters. He killed on order. He let go of everything; his morals and sense of humanity was tossed aside for the power that his Lord originally promised him with.
He ends up betraying him when he learns what he intended to do with Lily. At the potential of her danger, emotions he forgot he had came back to him; care, love, regret, and everything in between. That is the simplified version of why Snape starts working for Dumbledore.
From that point on, he walks where angels themselves feared to tread. At any moment, he could be discovered and it could be said without doubt if he was, he would die a horrible and painful death.
Snape doesn't care. For Lily, he would willingly go to hell and back.
He goes to sleep wishing to dream about her laughter. When he closes his eyes, it is her brilliant green orbs that he sees. When he first lays eyes on their child, he doesn't see James Potter first like so many others. He sees his Lily in the boy's eyes; he sees the woman he has always loved.
He always stays in the background, giving silent help to Potter when he needs it. He never intends to be a hero. He doesn't want recognition from anyone other then her. If he dies known as a heartless man and an enemy to Dumbledore's followers, then so be it. He could care less about things like that—they are all trivial matters when compared to Lily.
On his deathbed, the Potter boy is there. He tells him to take his tears and go to the pensieve. He asks him to look at him.
When he looks at Harry Potter, he does not see his father, his enemy. He remembers a red-haired girl, his childhood, and a time back when everything was simple.
The Potter boy is half his enemy, and half is love. The only woman he has ever loved chose the man that personally made sure his life in Hogwarts was hell; and though it is not with him, he is happy that she is happy. The outcome of their future, of her future, was a little boy with his mother's eyes. He had not died in vain. He had protected something that was a part of her, someone she was willing to die for. He had honored her in the only way he knew how to. In a way, he made amends for his past deeds.
He tries to say it all. Not to the Potter boy, but to Lily, who is sure to be watching down on her only son right now, at this very moment. He wants her to know what he has done, because a part of him still craves for her acceptance, even after all these years.
"You have your mother's eyes."
When he closes his eyes, he thinks he can see Lily Evans (not Potter), calling to him, surrounded by wildflowers that don't even hold a candle to her beauty. She laughs with him, and it is just like old times when they were both children, young and innocent. Birds are singing a happy, distant tune, and the sky is a serene forget-me-not blue.
He would never be what he wants to be for her—that spot was already filled by a certain messy-haired boy. But perhaps, just maybe, he could still be a part of her life. It's long overdue, he had to wait years for this day to come, and it's probably all in his imagination, but Lily Evans looks at him the same way she used to when they were friends. It seems a lifetime ago; he hadn't been able to clearly envision that smile in years, but now, as he is dying, he finds that can remember everything with clarity. Her memories come so easily that he can almost fool himself into believing that it was only yesterday when she smiled at him, not decades back.
It almost makes dying worth it—he thinks it justifies the cruelness of his situation, going as the man who lost a love to an enemy and was bitten to death by a monster while protecting her child.
He doesn't try to hold onto the tiny shred of life left in him any longer;
Lily Evans is calling to him.