He'd only wanted to teach the pathetic brat a lesson. He'd wanted to watch the crippling disappointment on his face as he realized that he'd been foolish enough to trust a Marauder.
He wanted to indulge in frightening the git until he pissed himself, and then send him running, terrified, back to his dorms.
He hadn't expected Snivellus to fight back, or for himself to get so furious at the brief show of defiance.
And it wasn't until after he'd opened the door and transformed that he realized what he'd done.
He could smell the complete and utter terror falling off the boy in waves. His heightened eyesight saw the tracks that the tears made down Snivellus' stricken face as he blindly searched for the tunnel entrance. He could see and smell the blood from the scratches and gouges in the Slytherin's skin, but-oddly enough- didn't smell any pain coming from the boy.
Either he was in shock and didn't feel it yet, or he was so used to pain that he could just easily ignore it.
And he could only watch, shocked at himself and his foolishness, as the werewolf- his poor friend, Remus Lupin, not in his own mind- stalked over towards the oblivious youngling.
Sirius bounded forwards, intent on stopping his friend from doing something he'd regret, but he was batted aside with great force and speed. He hit the opposite wall from Severus with a yelp and a whine as he felt one of his ribs crack.
It was obvious that the werewolf was not going to be deterred. Sirius could only watch in horror as Moony leaned over the frozen body of Severus, take in a deep breath, lick his chops, and exhale in apparent approval. It had been so long since the werewolf had been able to feed on human… and he'd never had the delight of a child. The blood was just so young. So pure.
And when Severus turned around, and Sirius caught sight of that terrified, tear-streaked face, and Severus screamed, Sirius could only close his eyes in terrible, gut-wrenching guilt.
And that small act didn't seem nearly enough.