It was a Potter who started it. James Potter, he of the cruel tongue and vicious mien. He rips away Severus' only friend, he marries her, and finally kills her. Yes, Severus is under no illusions as to who is truly to blame here. Had he not shamed Severus on that horrible, fatal day, Severus would not have lost his temper and irreversibly severed his friendship with Lily. Had he not married her and spawned that foul little brat – Lily's baby, little green-eyed child of prophecy - the Dark Lord would not have killed her. But Severus can tell this to no one, no one at all, because in their eyes, it is all his fault. And so Severus mourns, and spies, and waits for the day when he can enact revenge upon all who wronged him.
Except the day never comes. Attack came, not from his jealous Death Eater cohorts, not from his vengeful Hogwarts colleagues, but from an insignificant Hufflepuff third-year, in the guise of Veritaserum in his food. Or perhaps he misspoke. Attack came from all sides, and the little third-year was just the final weapon. The little girl, though angry and blaming Severus for her parents' death, could not have procured the Veritaserum on her own, after all. And Severus, cautious and paranoid in his spy role, would not have fallen into the trap were it not for the machinations of the crazy Lestranges. In the end, the Dark and the Light worked together, for a brief, unknowing moment, to betray Severus, the traitor.
But no, he forgets one more player.
In the end, it is Severus' own words, Severus' own carelessness, who betrays him, the spy.
And when he is dragged, broken and bleeding, to the Veil under the Dark Lord's amused gaze, he can only choke on his blood and laugh at the irony, the bitter irony, of escaping his torment on this mortal plane, only by joining Black in his shadowy grave. And in the end, the only thing he can think of, the only thing he can cling to, is Albus' promise that the Dark Lord's power would end one day, by the hand of Potter. And he laughs, laughs as he is shoved through the Veil, laugh as he spins into dark oblivion, when he thinks of the Dark Lord, his murderer, his tormentor, being defeated by a Potter.
How lovely, Severus thinks, as at the last moment, as the Veil reaches for his body and envelops him in Death's embrace, he makes one, final effort to Apparate away from the menace. He fails, and laughs again as he falls. How lovely to know that it will be a Potter who finally ends it.
But then, such is the story of his life.