I think he is a particular kind of hero. Not a global hero yet an hero for the pirate world
hero for pirate world is ultimately a pirate...lol...
see this scan
heroes are an ally of justice but he said he doesn't care about justice or anything...
more importantly for him being pirate means being outside of rules and restrictions i.e. beyond the law
As Luffy will be the first to tell you, he’s not a hero; at least, he’s not the classic type. While he doesn’t go out of his way to hurt anyone (unless they really piss him off), he doesn’t care all too terribly how his actions or lack thereof might hurt people. At several points in the East Blue saga, he points out that he’s not especially interested in whatever conflict is going on and wouldn’t have bothered to help until he came across someone who earned his respect to the point that he decided to intervene for his or her sake specifically.
When he breaks into Impel Down to save Ace, he also ends up also breaking out a lot of legitimately horrible criminals who would actually pose a serious danger to the public in the process of doing so, and this is even pointed out to him a few times; I don’t think it’s ignorance on Luffy’s part or anything, it seems as though he knows full well what he’s doing and just doesn’t care. When he punched out the World Noble, he knew full well what the consequences would be, even as he was doing it, but again, he didn’t care.
Basically, instead of seeing piracy as ‘freedom to attack whoever I want and take their stuff’, Luffy sees it in broader terms, as ‘freedom to ignore whatever boundaries or laws might hinder my ability to do what I want’, and he does take full advantage of that. He’s just not all that interested in attacking people who haven’t done anything to deserve it.