He'd wanted to keep his cool, to be bigger than this, but the second Snape had put James and Lily's names in his mouth he couldn't help himself. The pain of their losses was still fresh in his heart, the years he'd spent in Azkaban only making the wounds fester and eat at him rather than allowing any opportunity to heal. And when Snape pressed at it he couldn't help the reaction, sudden and angry that he of all people would dare to speak to him about the deaths he himself had caused.
"And you served her up to your dark lord, so eager for someone to want you, for someone to care." He spat the words harsh and angry heedless of the curse that had already been cast. He had more to say, more words for this supposed hero who didn't know a thing about what had happened that night - but a second spell was cast, and it wasn't his own, Sirus felt it - the sudden sharp bright pain and he didn't want to bother with words any more.
He moved his wrist in a sharp motion, wanting Snape to feel even a little of the pain he felt at the loss of the people he loved most, at the loss of his life, of everything that mattered, and of the pain that he now felt too, blooming hot and sticky across his chest before he was stumbling back lifting a hand to his chest. "You don't know anything." He panted.