Detention.
SIRIUS
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McGonagall had given both of them a detention that evening. It wasn’t great timing - it was the night before a full moon, and usually Sirius helped Remus with his homework on those nights, so he wouldn’t feel so behind while he was recovering in the days that followed. Well, Remus would understand. He was always up to give Severus what he deserved, and sometimes getting caught by a Professor was the price. Sirius and Snape were to work together to clean centuries-old gunk out of abandoned cauldrons in a storage room. They worked in silence, glaring at each other when their eyes met, and flipping rude gestures when the other’s back was turned. McGonagall returned to the room periodically to make sure they wouldn’t have another go at each other.
Sirius was scrubbing furiously at a thick, sticky green substance lining the walls of a cauldron. “You know what, Snivellus, this detention still isn’t nearly as gross as your greasy hair.” Sirius scooped up a handful of the gunk and lobbed it at Snape’s head. It landed perfectly, and started oozing down his ear.
“Watch it, Black.” Snape snarled, rubbing furiously at his hair. “You don’t have Professor McGonagall to save your sorry ass now.”
“Me?” Sirius questioned incredulously, “Come on, Snivellus. You wouldn’t have stood a chance against me without your death eater boy band there to back you up.”
“I could take you anytime! I’m better than you and you know it! Even I have more dignity than a little fag blood traitor.”
Sirius froze. His blood ran cold.
“Yeah, Black. Everyone in Slytherin knows about how your boyfriend got you kicked off the family tree.” Snape spat aggressively. “You can keep pretending you’re so tough all you want, but everyone knows the truth.”
Rage bubbled up in Sirius. He was red hot, and ready to explode.
“You don’t know shit, Snivellus! What, you think it makes you better because I have people who actually care about me?” Sirius was practically yelling now, fury spilling out of him from all angles. “You have no-one! Your family hates you just as much as mine does. But at least I have friends. You had one, but you didn't deserve her. You blew it, Snivellus.”
Sirius fixed his features into the cold, cruel expression that his own mother wore so often. “I know you love Lily. But guess what? She hates you. She told me so herself, earlier today. Lily hates you, and it’s all your fault.”
Sirius could tell he hit the target. Snape’s face turned pale, and he looked like he was going to be sick. But Sirius was still overflowing with wild emotional turmoil. His parents’ abuse. The danger to himself and Remus. Lily, crying hysterically in the bathroom. Snape taunting Regulus. Regulus blatantly discarding Sirius. It was all too much, and it pushed Sirius over the edge. His rage blinded him. He couldn’t think clearly, and he just kept spitting fire at the other boy.
“You’re a weak, slimy git, Snivellus. But if you’re so convinced you could take me in a fight, prove it. I know a place where no professors would interrupt us.”
Prove it. Snape’s fatal flaw; he was always so desperate to prove himself to others. As expected, he took the bait, and walked right into Sirius’ trap.
Sirius relished in the hatred painted across Snape’s face. For a moment, it was like looking into a mirror.
“There’s a hidden passageway beneath the Whomping Willow, on the far side of the grounds. You have to tap your wand against the knot on the base of the tree three times to open it. Meet me there at sun down tomorrow, if you have the balls to show.” Sirius spit at Snape’s feet and calmly strolled out of the room, detention be damned. He’d finally had the chance to release all his hatred and anger, and on his favourite target, too. Sirius wasn’t angry anymore. Snape had been an excellent emotional outlet, and Sirius felt like he was flying, riding the wings of his vindictive satisfaction.