@The:
He would still destroy an island–in turn, kill people--if it came to meat. It's just as bad; it's still killing (just in a different way).
He didn't say that. The line is that he would understand if she was going to destroy the island over meat, not that he himself would actually do it.
Also, did none of the Fishman think that maybe giving all the remaining sweets to Luffy was a bad idea? I know they saved the island and all, but it seems like the agreement with Big Mam is very well known. Someone should have said "Maybe its a bad idea to give away our protection money". But hey, plot and all that.