User blog comment:Monkey.D.Me/OPSI case 8 Who killed Queen Otohime/@comment-1784001-20110531193611

I highly doubt that Saint Mjosgard is a suspect. As far as I know, he already returned to Mariejois. Whoever shot Otohime also burned the signature box and I am certain that the culprit is someone who is against co-existence between fishmen and humans so Hordy Jones is my number one suspect.

Besides the World Nobles do have their own motives to support Otohime's cause. We all know that they don't really care about anyone except themselves. When fishmen and merfolk migrate to the surface, it would be more easier for the nobles to enslave them and obviously Otohime did not took that into consideration. So someone on Fishman Island tried to prevent that from happening by having Otohime killed and by burning the signatures.

As for Decken, he was only focused on getting Shirahoshi and nothing more. I am pretty sure that he didn't need to burn the signature box and killed Otohime. Shirahoshi was already out in the open so Decken could just walk by her and touch her.