Mysterious Wonder

"A wretched man, he trained in the art of self defense... then used it to kill his entire family. As a baby he never once smiled nor cried. His family must have loved him, for he treated them horribly and they did everything they could for him. Many girls had crushes on him. He hunted them down as well. My father got involved with him in a dirty buisness that none of us knew about, apparently he hired my father to assasinate a man for a hefty price. My father failed to do so due to his moral beliefs, so in return he slaughtered most of my family- I was away at the springs the day it happened. He most likely does not know that I am linked to my father, the man he killed."
This sounds right, you can't help feeling like you worked for him once yourself... but as soon as that thought crosses your mind you shake it away. You'd never work for a monster who kills his own kin.
"His weakness is his sword." You blurt out. She looks at you in surprise.
"Excuse me?"