Werewolves. I believe that Severus' antipathy to werewolves was due to something he believed to be Remus’ betrayal. Remus had never tormented him. That would have meant a great deal Severus, as a lonely teenager. There are two times we read about Severus following someone and spying on them for his own ends. One is Lily, in the playground, (which naturally implies more such incidents.) The other is his spying on the Marauders.
The books speak of him hiding in the bushes, watching Lily with hunger. He was not a predator, but despite everything, despite planning and cunning, he was something much more innocent. He wanted a friend.
I believe it was in the same spirit that he watched the marauders. It is true, he was amongst a gang of Slytherins, who nearly all turned out to be Death-Eaters, but they
were Slytherin. They were the powerful rich old families, pure-blood, wanting for nothing. The demands put on the neglected boy we see on the train must have been enormous, and they would have had to be hidden, must be demonstrated immediately even as they were developing.
How lonely he must have been, would have been, even had Sirius, James and probably others, not tormented him while their followers jeered.
He would have noticed Remus with them, shabby and poor as himself, another boy who would have been an outcast. He would have seen the apology in Remus' eyes, the awkwardness, the inability to prevent his friends, the disinclination to participate. Intrigued, he would have watched, noticed the illnesses, the absences, perhaps scratches and minor wounds. In his mind he would have conjured sympathy in Remus - not pity, but possibility of understanding, fellow-feeling, even... even friendship.
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