Severus saw her once after the wedding, strolling through Diagon on his arm. James' final victory against him. To steal the woman who should have been his. His friend, his wife, the mother of his children. Watching her, he realized what he needed to do. He approached the two, "hello Lily, James." Lily smiled and James nodded curtly.
"We haven't spoken in so long," he addressed Lily, "are you free this Sunday?"
"Of course," she said and Severus was glad to see a frown on James' face.
That Sunday he and Lily met at a restaurant in Godric's Hollow. As they were leaving he stunned her and Apparated to Spinner's End.
"Enervate," he whispered. He saw terror in her eyes as she realized what had happened.
"Sev," she begged, "please don't."
"Don't call me that. You don't deserve that right; you don't deserve my love, not now that you've left me for him. Potter? He's a barbaric imbecile, he could never love you like I do," he drew the knife, "You betrayed me. I loved you and you shunned me. Why don't you try a taste of my pain?" and he began to cut. They started shallow; Lily took each one with shock and tears as bright red blood spilt from her body. Then he cut deeper, and faster. His anger began to attack her violently. He wasn't sure when she died, but when his anger ebbed all that was left was a corpse, so mangled as to be hardly recognizable as human, and a pool of blood on the floor. He transfigured what remained of the body into a dark pink lily and left it floating in the blood. Then he disappeared, never to return.