Your parents drunkenly kicking you out, leaving you vulnerable and alone. You found a sword, but it was dull and rusty. Though it was something.

So when a Fatui Harbinger tried to manipulate you into their trap, you killed them with that dull blade. You didn't even know you could.

So now, you ended up in the hands of the Fatui. Arlecchino, the 4th Fatui Harbinger, decided to question you.

"Hm. You're pretty interesting."