I've been thinking on this for a while, and I know it's entirely stupid to be thinking about.
In technical terms, pirates are classified as 'bad' while marines are classified as 'good'. Now this is just in technical terms of the names themselves, not those who hold them.
Though I can quite easily say that Crocodile is as bad (in the awesome way) as you can get in my mind. Holding a vice grip on an entire country by holding a monopoly on the water? Ingenious.
But if we apply the technicalities of these terms in a literal sense to the One Piece world, we end up with a jumbled confusion. This happens because Luffy and the protagonists fall under pirates, who fall under 'bad'. But we know that the Straw Hats and -some- of the pirates are 'good'. And the marines are basically the antagonists, despite their name falls under the title of 'good'.
I know I've been thinking too hard on this, but what does one do with the definition in such a case? Pirates by definition are bad. Marines by definition are good.
What to do…I'd like to get what you guys think on this little title placement.