@boiga:
Well argued, and the more long and ramble-y the better. Although I regret that the application of logic to an issue like this makes playing devil's advocate so much more difficult….
This situation seems more complex than previous ones seen in this series though. When the conflict between humans, human collaborators, and the antihuman fishmen supremacists comes to a head, I have a feeling that Luffy will boil it down to "Arlong was an asshole" and leave it at that. I don't think this arc can end in the traditional "beat up the bad guys and the good guys throw a party to make up" ending seen in alabasta and skypeia. The moral picture is more muddled.
Personally, I'd agree with Luffy on his sentiment. Yes, as mentioned, there are circumstances that can help explain Arlong's actions, but to me, it ultimately boils down to "he was a bad (fish)man". Still, that's a matter of opinion, and I don't want to start bringing it up again, so will leave it with that.
On your main point, though, I'm in complete agreement. Arlong specifically aside, the situation as a whole is definitely incredibly morally ambiguous, purely because the history of discrimination certainly can justify quite a variety of reactions, even if opinion on just what extent, and whether it includes Arlong, varies. Fishmen being generally distrusting and even racist towards humans, for example, could be understandable; especially if you consider the possibility of some who have never interacted with a human in their lives, and had only grown up being told stories of how all humans are complete irredeemable demons whose only thought in life was to subjugate and humiliate mer-kind.
We'll have to see what happens, of course, but I can definitely seeing this arc heading for 'tragic villain' territory. The potential pathos, to the reader, of having their hero have to defeat someone, knowing they have no other choice, but also knowing enough that they'd wish above all else that there were another choice, could be quite powerful, pulled off well, and I have no doubt it will be should Oda choose to go that path.
On the other hand, it's possible that sort of conflict will be glossed over by introducing a third party that ends up making the Strawhats and otherwise antagonistic fishmen set aside their differences. Actually, in a sense, there's precedent for it. Consider Skypiea, but pretend that Enel hadn't been a factor, and rather the villains of the arc had been the Shandians. All else equal, then, we would have had more or less the situation I described above: as readers, we would know that the Shandians were simply fighting for their ancestral home land, and all the tragic back story involved with that, and wouldn't help but sympathize with that. At the same time, we would have to accept (assuming, still hypothetically, that it were written this way) that defeating them was sadly the only choice. Of course, that wasn't the case: instead insane villain Enel was introduced, and in the process of defeating him, the differences between Skypieans and Shandians ended up being settled.
So, again, not to say things will happen in one particular way or another, but I would just say that while I agree the situation as a whole is in no way black and white, how much Oda will choose to really delve into that is something I'll personally just wait to see.
Oh, and thanks all for the welcome, appreciate it! I don't exactly consider myself a new comer… I've been around for quite a while, just never posted before. Still, nice to know that my choice to stop lurking seems to be considered a positive effect!