It was a cold, dark midnight when Sirius Black made his decision. It was standing there, in front of his best friend's limp, lifeless, broken corpse, beside the ruins of his house that he decided—he was going to find Peter Pettigrew and make him confess. He was going to find Peter Pettigrew—and kill him.
Sirius looked at the crouching, terrified, ratty man only ten feet away with what some might call a deranged look in his dark eyes. His entire being was filled with revenge; he was going to KILL this man, this man who had caused the deaths of his best friend and his best friend's wife.
"Having fun, Peter!" Sirius called out, his voice higher than normal. "Are you ready to come out and play... come on... you knew you could never match me or James in duelling... IS THAT WHY YOU KILLED HIM, PETER?" Sirius let out a mad, barking laugh. The light in his eyes was no longer a glow, it was a burning, fierce, demented desire for murder. It was bloodlust.
"I didn't kill him, Sirius—NO!" Peter was screaming in terror and Sirius was relishing every minute of it. He was going to be his cousin for a minute; he was going to play with his food, and then, he thought with a sick, demented grin, he was going to eat it.
Grief and despair ran alongside the bloodlust and anger in Sirius' veins and that made him all the more dangerous as he screamed, "DON'T LIE TO SIRIUS BLACK, WORMTAIL! You always knew we could see through your lies—were you tired of being least best? Tired of being second to James and Remus and I? Is that why you went to Voldemort? So you could feel... special?" He said the words with utmost contempt.
"Sirius, please, you have no idea the weapons—he tortured me, Sirius!"
Sirius was not amused. "ONLY BECAUSE YOU TOLD HIM YOU WERE SECRET KEEPER, YOU BASTARD!" The words flew out of Sirius' mouth without any conscious command from their master. "ONLY BECAUSE YOU LET HIM! YOU SHOULD HAVE DIED! MERLIN KNOWS YOU DESERVE IT!"
"Sirius..." Peter whimpered, pathetic, pitiful- But Sirius was pitiless. "Sirius, please, I'm s-sorry... Come now, my old friend-,"
"You are no friend of mine," Sirius said, despise in his voice, hatred in his eyes. "You may have escaped Voldemort killing you, but you won't escape me, Peter. It's over."
The force of the blast knocked Sirius off his feet. He slammed his eyes shut to protect them from the grit flying through the air, and when he looked up he saw the corpses of Muggles, the crater in the middle of the street, and Peter was gone. For one split second, he thought he had killed him—and then he saw the disembodied finger lying, half-buried under rubble—and then Sirius understood.
As the Ministry's Aurors clasped his arms in strong hands and began to Apparate, Sirius Black did not just laugh—he cackled.