the_debt_of_time

"Because," Remus began, ignoring Sirius's outburst, "Professor Snape and your
father were long-time rivals. Slytherin versus Gryffindor. House prejudices played out
much like they do now."
"It had nothing to do with House rivalries," Sirius interrupted. "He started it."
"You see Harry, Sirius played a trick on Snape which nearly got him killed," Remus
said, eyeing his friend and taking note of the way that Sirius averted his gaze. "A trick that
involved me, a full moon, and this very shack."
"Oh, he deserved it. At least a little bit," Sirius grumbled. "He was always sneaking
around trying to get us expelled for one thing or another, and he threatened.. ." He
growled and let himself trail off as his attention flickered to Hermione.
"Sirius tricked Snape into going beneath the Whomping Willow. Had he gotten as
far as this shack he would have come face to face with a werewolf. Luckily, your dad found
out what Sirius had done and saved Snape just in time. Dumbledore forced Snape to keep
quiet about my condition."
"So that's why Snape doesn't like you," Harry said slowly, "because he thought you
were in on the joke?"
"That's right," said a cold voice from the wall behind Remus. "I told Dumbledore
you were helping your old friend into the castle. And here's the proof. Expelliarmus!"
"Brilliant!" Sirius said loudly after losing the wand in his hand. He applauded as he
stared ahead into Snape's black eyes with disdain. "And—as usual—dead wrong. Now give
us our wands back. Remus and I have a bit of unfinished business to tend to."
"Give me a reason," Snape dared, approaching Sirius swiftly and sticking the end of
his wand against the man's throat. "Give me a reason to do it, and I swear I will."
Sirius was shaking with obvious hatred as Snape glared into his eyes.
"Don't be a fool, Severus!" Remus snapped.
"He can't help it. It's habit by now," Sirius mocked.
"Shut up, Sirius!"
"Listen to you two," Snape sneered at them. "Quarrelling like an old married couple.
The creature and the criminal."
"Piss off!" Sirius spat.
"Professor Snape," Hermione quietly said from the corner of the room, her voice
small. "It wouldn't hurt to hear what they've got to say, w-would it?"