Ele amava as Artes das Trevas e entrou com a turma errada.
As lembranças de Snape como garotinho de Sirius confirmam que ele era obcecado por magia negra. Esta foi a base para sua associação com os Comensais da Morte.
"Snape’s always been fascinated by the Dark Arts, he was famous for it at school. Slimy, oily, greasy-haired kid, he was,” Sirius added, and Harry and Ron grinned at each other. “Snape knew more curses when he arrived at school than half the kids in seventh year, and he was part of a gang of Slytherins who nearly all turned out to be Death Eaters.”
(Goblet of Fire, Chapter 27, Padfoot Returns).
Os amigos de Snape também tiveram um grande papel a desempenhar. Eles pegaram o que já era uma fascinação muito distorcida com a Magia Negra e a exacerbaram. Eles foram uma má influência sobre ele, incitando-o mais e mais no caminho para a adesão total do Comensal da Morte. Snape era muito cego em relação a como Dark a magia de alguns de seus amigos era.
"... thought we were supposed to be friends?” Snape was saying. “Best friends?”
“We are, Sev, but I don’t like some of the people you’re hanging round with! I’m sorry, but I detest Avery and Mulciber! Mulciber! What do you see in him, Sev, he’s creepy! D’you know what he tried to do to Mary Macdonald the other day?”
Lily had reached a pillar and leaned against it, looking up into the thin, sallow face.
“That was nothing,” said Snape. “It was a laugh, that’s all-”
“It was Dark Magic, and if you think that’s funny-”
(Deathly Hallows, Chapter 33, The Prince's Tale).
Este movimento para a magia mais escura e mais escura também foi o que alienou Lily de Snape. Quando ele a chamou de sangue-ruim, ela basicamente cortou a amizade do desespero pelo caminho que Snape havia tomado. Ela também o chama por fazer parte de uma gangue de simpatizantes conhecidos dos Comensais da Morte.
“It’s too late. I’ve made excuses for you for years. None of my friends can understand why I even talk to you. You and your precious little Death Eater friends - you see, you don’t even deny it! You don’t even deny that’s what you’re all aiming to be! You can’t wait to join You-Know-Who, can you?”
He opened his mouth, but closed it without speaking.
“I can’t pretend anymore. You’ve chosen your way, I’ve chosen mine.”
(Deathly Hallows, Chapter 33, The Prince's Tale).
Essencialmente, Lily foi uma influência sobre ele. Dark Magic e seus amigos do Comensal da Morte eram outros. Ele tentou manter intactas essas duas partes de sua vida, essencialmente jogando-as uma contra a outra. Mas eles eram irreconciliáveis. Ele escolheu os Comensais da Morte, para seu eterno arrependimento. JK Rowling abordou esse conflito interno em uma entrevista.
Nithya: Lily detested Mulciber and Avery. If Snape really loved her, why didn't he sacrifice their company for her sake?
J.K. Rowling: Well, that is Snape’s tragedy. Given his time over again he would not have become a Death Eater, but like many insecure, vulnerable people (like Wormtail) he craved membership of something big and powerful, something impressive. He wanted Lily and he wanted Mulciber too. He never really understood Lily’s aversion; he was so blinded by his attraction to the dark side he thought she would find him impressive if he became a real Death Eater.
(Leaky Cauldron Web Chat).