He had received his letter, and his mother showed genuine pride. "Soon son, you'll be like every Prince before you. You'll be a Slytherin!" the look in his mother's eyes made his heart sink. If he didn't become a Slytherin he would be stepping on the only thing his mother ever wanted from him. She was to take him to Diagon Alley the next day and his father would stay behind. As aloof and disgusted as he always was.
That night the voice came back for the first time in awhile.
Tell me, boy, what house do you think Lily Evans will be sorted to?
After a moment of thought Severus' heart sank. Lily had called his remarks about her sister mean, and that was just not because it was her sister. It's because she loved people. She had loved even him, someone who's own parents cared little to nothing about him. We can still be friends, though, Lily and I, and my mother will still be happy.
If you fancy Lily Evans, then let me tell you. Becoming a Slytherin will make you into a man she hates. Can you see that girl ever loving a man who would follow a murderer? You know nothing of love, boy. Just as she will learn not to care for you if you become a Slytherin, your mother will never love you even if you do.
Severus grew angry with the voice inside of his head. He was no longer faced with a simple decision. He wanted to stuff the voice out of his head. His hexes, his potions, he would show Slytherin house and his mother he was worth something with them and then- then he'd have respect. So much respect that even Lily would-
If you become a Slytherin, Lily Evans will die, and you will be the cause of it.