Call Them Brothers by Regina Spektor feat. Only Son.
The air fizzled and crackled with magic as curses and shielding spells bounced off each other. Remus furiously fired hex after hex, mimicking every nasty, cruel spell he'd ever seen his friends use on repulsive Slytherins, and the colourful light sparked from his wand as he whipped it at Sirius. The other man defended himself with panicked, frantic movements, seemingly unwilling to fire at his old friend. Remus blinked back angry tears as he continued his assault.
"You foul, backstabbing murderer!" His voice was a growl in the dark alleyway.
"No, hold on-"
"Shut up! " A Stupefy narrowly missed, and he hissed in satisfaction as Sirius ducked. "Stop lying to me! It's too late!"
"Moony, please, I didn't-"
"No!" He punctuated his furious scream with a body-bind jinx, which Sirius blocked. "Don't you call me that! Like we're at Hogwarts, like James is- like nothing's changed!" He stopped suddenly, clutching his head in his hands as loud sobs tore through his body.
"It's your fault," he croaked finally. "You did this. James! Your friend! Our friend! And Lily?! I loved you! I trusted you!" Refusing to look at the other, he turned on his heel and Apparated. Left alone in the alley, Sirius sank to his knees.