If you recognize it, it's not mine. Characters belong to JKR.
Why him? Why now? I could kill you if I wasn't exhausted, if the full moon wasn't last night. You told him, you told him, that odious, spiteful boy that has done nothing but argue and fight with you since day one. How could you? Was it really that trifle to you; it was a little more for me. And you told him! How could you? You, with an affluence of comebacks, sarcastic remarks, had to use the one that I gave you, though unaware of your contentious spirit. You'll do anything for the last word, won't you? I've tried so hard to hide it, keep it a secret, to remain plebeian in the classrooms surrounded by the enemy. It means nothing to you!
You've no idea how transient this time is for me but I do; I leave this school and nothing else matters. I'm nothing more than the monster mothers hide their children from; regardless of anything I do or say. And you walk over all of it, this precious time, for what? Heaven forbid you be deprived of one win in an argument; heaven forbid you think of anyone but yourself.
Why, Sirius, why? Did you think of the consequences, or did you think them destitute? Did you expect me to shrug this off, pretend it didn't happen? Did you ever imagine the schism this would cause between us? How could you, Sirius? You, who habituate arguments with him solely for the sake of the argument, how could you have thought about what you were saying, about how it affects others, affects me.
How could I have trusted you? How could I have allowed you to know my secrets, to know me? Why did I tell you? Why didn't I lie? How, in Merlin's name, do you expect me to trust you again?
But you've stopped listening, haven't you? When have you ever listened to me, especially when I expostulate with you against the most insane, idiotic, childish things? I'm through; I'm done; I'm leaving. Go ahead and cause trouble, cause pain, cause strife. It's all you know, all they've taught you, and you won't let anyone teach you otherwise.
Merlin knows I've tried, Sirius. I've tried to disabuse you, tried to break your stoic façade, tried to purge you of their malign and hate. But you don't listen; you barely care! Don't lie to me now: you told him. You told him, of all people, Sirius, him. Why, Sirius? Answer me: how could you do this to me?