Ok, we know who killed Otohime. As to why she was killed, that is still a little fuzzy. My question is this, who hired the pirate to kill Otohime? There is no way some stock character, run of the mill pirate would take the initiative of going to Fishman Island and killing someone on his own. Someone was backing him. My question, who? My guess is it was government sponsored, or at least by a World Noble. Mjosgard comes to mind. But yeah, this runs a little deeper than the flashback let on.