DISCLAIMER: I own nothing but the idea for this two minute flurry of fingers on keys. All characters and names are property of J.K. Rowling and whoever else happens to own the rights. There, now that's over with, on with the drabble!
'There wasn't a witch or wizard who went bad who wasn't in Slytherin.' That's what my best friend told me the first day I met him. We were on our way to Hogwarts at the time. Then I believed him, mostly based on my perceptions of those that were destined to be both Slytherin and those destined to be Gryffindor. It was two years before my childish beliefs were shattered beyond all recognition.
I was hiding where I wasn't meant to be during my third year, in the Leaky Cauldron during a Hogsmeade visit that I didn't have permission to attend. I heard a conversation I shouldn't have heard. A conversation about my father and his friends. About how Sirius Black betrayed James and Lily Potter to Voldemort and then killed Peter Pettigrew and thirteen Muggles in cold blood. I believed this as well. Luckily for all involved I didn't believe this for very long either.
At the end of that year I found out the truth. It wasn't Sirius Black, the only member of the House of Black to ever be a Gryffindor, who betrayed my parents. No, it was Peter Pettigrew, fellow Marauder and loyal supporter of Voldemort. A Gryffindor. It was no Slytherin that turned on my Muggle-born mother and my father. It was a Gryffindor. If all Dark witches and wizards were in Slytherin, then Pettigrew's school years were a lie.
But that is all in the distant past. Sirius is dead, slain by his own cousin in a fight that I foolishly drew him into. The woman who killed him is also dead, outmatched by a Slytherin who turned from the path his fellow students had taken. In the end it was a Gryffindor that dealt a blow more grievous than any that Wormtail had struck. My best friend, the second youngest of a long line of Gryffindors, had joined with my worst enemies. My best friend killed my wife and newborn son, his own sister and nephew.
If all Dark Wizards come from Slytherin then my entire life has been a lie. If two Gryffindors can turn on their closest friends then more can as well. And what of the Ravenclaws and Hufflepuffs that supported the Dark Lord. I saw them in the Battle. I killed any who stood in my way. My message is that anyone can turn to the Dark Arts. They don't have to be Slytherin to be evil.