получается, он в 16 лет уже настолько инфернальный мудак, что попытался убить одноклассника, используя при этом своего типа-друга?

"Professor Snape was at school with us," said Lupin. "He fought very hard against my appointment to the Defence Against the Dark Arts job. He has been telling Dumbledore all year that I am not to be trusted. He has his reasons... you see, Sirius here played a trick on him which nearly killed him, a trick which involved me—"

"It served him right," snarled Sirius. "Sneaking around, trying to find out what we were up to... hoping he could get us expelled..."

"Sirius thought it would be — er — amusing, to tell Snape all he had to do was prod the knot on the tree-trunk with a long stick, and he'd be able to get in after me. Well, of course, Snape tried it — if he'd got as far as this house, he'd have met a fully grown werewolf — but your father, who'd heard what Sirius had done, went after Snape and pulled him back, at great risk to his life... Snape glimpsed me, though, at the end of the tunnel. He was forbidden by Dumbledore to tell anybody, but from that time on he knew what I was...."