Sirius was well-schooled in hatred, to hate muggles, mud-bloods, blood traitors, and half-breeds – and he had rebelled, hadn't he? If the gold-red on his robes means anything, it means not-Slytherin. It means the antithesis of silver and green.
Severus Snape is greasy and cruel and deserves it. Sirius tells himself this, as he tosses out a lie like a chocolate frog and smiles hard with his mouth. Snape's eyes widen and his nails dig crescent moons into his palms.
The Whomping Willow, midnight, Sirius says. But you won't come, you're scared of us, you're scared to death of James, aren't you? Sneak. Coward. Snake. Bet you won't come. Bet you. Dare you.
The red of Snape's anger summons the blood from his face and when Snape responds his voice is tight, lustful, utterly sure: I will.
What a Gryffindor, a part of Sirius smirks.
Some poisons can't be expunged.