Complex sentence: Brief explains to Luffy that the Baron's crew were all killed in a storm and that he was the only survivor, who then started hateing all close-knit pirate crews and wishing to separate them.
Split into simpler sentences: Brief explains to Luffy that the Baron's crew were all killed in a storm and he was the only survivor. And that now the Baron hates all close-knit pirate crews and wishes to separate them.