He has often wondered what he might see in the mirror.
His first thoughts were of vengeance. He, Severus, spitting on the corpse of the Dark Lord? Watching a Dementor administer its kiss to Sirius Black? Or perhaps himself, lying in a pool of blood, slowly exsanguinating?
But then he thinks, whatever inclinations he has for revenge lack the ardor that all the books about the mirror seem to speak of. "The deepest, most desperate desire of our hearts." That's what the books had said. Deep? Desperate? These were not words that he would use to describe his life these days. Bare. Bleak. Hollow. Words that seem much more apropos. He pictures a mirror reflecting nothing, a Snape-less room, as if he were a vampire. (He already looks the part.)
After he left Hagrid's hut that day, he decided that he was being altogether too charitable with himself. He remembers the hill, the wind howling, himself kneeling in the cold, Dumbledore's voice dripping with contempt: "You do not care, then, about the deaths of her husband and child? They can die, as long as you have what you want?" And he thinks that's what he would see, him and Lily together, the Dark Lord officiating their wedding, right there in Godric's Hollow, in front of Potter's limp body.
He does not like the version of himself he saw in Dumbledore's eyes that night. He does not like any versions of himself, really, but this one he hates most of all. Because he fears it is the one that most resembles reality.
But this is not what he sees now, in the Mirror of Erised. He sees Lily; Lily alone. No trace of Snape the Avenger, or Snape the Vampire, or, indeed, Snape the Contemptible Bridegroom. There was only Lily; Lily as he last saw her, in Diagon Alley, not long after her wedding. She had invited him - he still has the invitation; in a box he never opens - but he didn't have the nerve to go. She was browsing books at Flourish & Blotts; he was on the street trying to make himself inconspicuous. She was wearing a green jumper and faded jeans, her hair tied up in a slapdash ponytail. He wanted and didn't want her to turn around and see him. She looked out the window and their eyes met for a second before he hurriedly turned the corner.
She looks at him now, just as she did then. Her eyes hold no accusation, but neither are they filled with longing. The edges of her lips are tugged upwards, not quite a smile. A wedding ring glints on her left hand.
This is it; the deepest, most desperate desire of his heart. Not murder or torture or lust. Just Lily, restored to the living. A simple undoing.
Severus Snape weeps.