Theory Vander Decken didn't go Crazy.

Vander Decken was the original Captain of the "Flying Dutchman", who was said to have gone crazy and killed his crew on his way to Fishman Island. But I don't think that's what happened.

Oda has done this many times before with the Rumbar Pirates, Jinbei, Noland, Fisher Tiger, and others when we hear about them with half truths and later getting the full story. Vander Decken is going to be the same.

My Theory.

Vander Decken made it to Fishman Island and actually befriended them, but back them Fishmen and Mermaids were still considered Fish and were taken by the humans as Slaves. Vander Decken was said to be Crazy because he sided with the Fishmen and protected them from the humans.
As for him killing his crew, three possibilities. One they died on the way to Fishman Island because of how dangerous simply going there is. Or two they tried to protect Fishman Island with Vander and died and he got blamed for their deaths, or three they wanted to sell Fishmen and Mermaids as Slaves and Vander killed them.

Even what was passed down to his decedents about the Mermaid Princess was probably twisted over the years. Instead of "you must have the Princess" it was probably "you must Protect the Princess".
Or maybe, He sacrificed his own Crew to summon Sea God or gain his Trust
And He was promised to be brought back once New Poseidon appears