So its unknown who killed Otohime at this point, though Decken is a prime suspect, having been at the scene.
The info about Shirahoshi is the really interesting part though. Especially about "someone who will guide her"
The obvious thought is that this refers to Luffy (which would be a nice addition to the "The person in the straw hat will destroy fishman island" one).
Random thought: Luffy convinces her to use her powers to move fishman island to the surface. Thus he "Destroys fishman island" as it is now, and leads her to change the world, by actually starting to have her mother's dream come true. (An alternative to the obvious "NEXT STRAWHAT CONFIRMED!" thoughts which will obviously pop up after this chapter.)