It's not that difficult to see Hody's POV at this point. He doesn't want to risk the Strawhats winning the battle on the island, so he noticed a drastic and quick solution to kill then all in one stroke so that only fishmen and merfolk survive. He was pissed earlier because Decken could have killed him with the Noah attack while he was on the island. I'm not saying Hody is making a sane decision–it's akin to Nero suddenly deciding to burn down Rome so it could be rebuilt to how he saw fit. I'm not sure if the island could possibly be rebuilt, but it seems Hody was going to start anew from the underwater Fishman district.
That is if he and the New Fishman Pirates didn't
! suffer from the side effects of the energy steroids and transform into decrepit fossils. Hody's dream was probably doomed from the moment he started to consume those cursed steroids