Luffy's first bounty exceeds Arlong's current bounty.
Buggy departs, saying "Sorry kiddos, I'd love to make things right, but it's time to exit stage left." Traditionally in theatre, stage left is used for the villain's entrances and exits (and stage right for the heroes), so Buggy is acknowledging that he's not a good guy.
Luffy brings down Arlong Park because he can't wound Arlong directly. He is doubly angry and wishing to destroy the place Nami was kept prisoner.
Buggy isn't among those Luffy voices disappointment in when insulting Arlong because Buggy helped them (reluctantly).
Nami joins the Straw Hats in the attack on Arlong Park, unlike the original where she arrives later with the villagers.