All Blacks go Mad
It was rumoured and said to be proven that all Black's go mad, "Just look at Bellatrix!"
Some thought it was the position of Lord or Lady Black that finally drove them around the bend, while many others thought it was in their blood.
Sirius had watched his aunts, uncles, grandmothers, grandfathers, cousins, brothers, sisters and even his mother and father go insane (although he privately thought they'd been insane for far longer); each of them in their own little way. Bellatrix had been the worst when it finally hit her, he recalled. She had killed, robbed and tortured her way through an entire muggle city for the fun of it.
It had been rumoured around Hogwarts and the Ministry of Magic for years that Sirius would continue the Black insanity tradition. There were bets and the way they would watch him, waiting for him to finally snap; he'd heard the rumours. And here he was, proving them right, laughing and laughing as they carried him to Azkaban.
From a nearby sewer, a rat with beady little eyes and a missing toe watched the scene. He had been right, he thought, Sirius Black had been the perfect cover. After all, who would suspect the coward Peter Pettigrew when they had the last living member of the Black family, a family notorious for its alignment with the dark and with each and every member going insane.
Slightly chuckling to himself in the way only rats can, he scurried away quickly; hoping to implement the next phase of his master's grand plan as soon as he could.