Where did you read that claim? Melos was a colony of Sparta, so there would have been formal and informal connections with Sparta and it's not a wild idea that some Melians had contributed to the war. Thuc. does say that the Athenians and Melians had become hostile to each other, if not yet outright belligerent.

About the destruction of Melos being more horrible because it was neutral, that was something I didn't understand until for completely different reasons I had to make a presentation on ancient navies. Thuc. does make the Athenians seem like real assholes there. I'm not saying the Athenians were not real assholes, but there's something we miss because we don't know what Thuc. knew.

We are used to navies being made of up ships that are more or less self-contained. Aircraft carriers with enough power to be replacement generators for entire cities, submarines that can remain submerged without even the need for air for more than a year, and so on. We have the notion (I don't know whether this notion is factual) that a navy group can operate more or less anywhere wet with little support.

That's not how ancient navies worked. Triremes usually didn't carry supplies, and there was no logistical system for supplying them. Crews purchased food daily from markets either in a port, or that popped up near wherever the ships beached each night (there are exceptions to this, but just leave them aside for now). That is, unlike modern navies (at least in the imagination), ancient navies were limited in their area of operation by their ability to obtain support from land in that area.

As a neutral, Melos was free to trade with both sides, which meant Peloponnesian ships could use the markets on the island. Because it's in the Cyclades, simply by being neutral it provided the support necessary for Peloponnesian operations against Athens' island subjects, from which Athens derived its ability to withstand a siege.

Thuc. does talk about this, when he makes the Athenians say they need to either get Melos to join the Athenians or destroy her because they are afraid of the revolt of Athens' subjects - it's through Spartan intervention that Athens' subjects detach themselves from the empire. Thuc. doesn't make the strategic problem of a neutral port in the Cyclades explicit, I guess, because the whole episode is set up as an abstract investigation of the problem of empire. Thuc. uses the idea of a free state as an ideological challenge to empire that the empire must not allow its subjects to think possible.

I think. It's hard to tell what Thuc. means to do with just about anything.