This is a very good question that gets at some interesting questions regarding Greek identity and, of course, our sources. Which gets to the heart of it, because we do not actually know why Ephialtes did what he did, perhaps he was a genuine Medizer, perhaps he simply saw the opportunity for a quick buck, perhaps he had some sort of personal grievance, we can't say because this is our best information (from Herodotus book 7):

\213. The king being at a loss how to deal with the present difficulty, Epialtes son of Eurydemus, a Malian, came to speak with him, thinking so to receive a great reward from Xerxes, and told him of the path leading over the mountain to Thermopylae; whereby he was the undoing of the Greeks who had been left there. This Epialtes afterwards fled into Thessaly, for fear of the Lacedaemonians; and he being so banished a price was put on his head by the Pylagori when the Amphictyons sat together in their council at Thermopylae [ed: Malis was a member of the Amphictyonic league]; and a long time after that, having returned to Anticyra, he was slain by Athenades, a man of Trachis. It was for another cause (which I will tell in the latter part of my history) that this Athenades slew Epialtes, but he was none the less honoured for it by the Lacedaemonians.

\214. Such was the end of Epialtes at a later day. There is another story current, that it was Onetes son of Phanagoras, a Carystian, and Corydallus of Anticyra, who spoke to the king to this effect and guided the Persians round the mountain; but I wholly disbelieve it. For firstly, we must draw conclusion from what the Pylagori did; they set a price on the head of the Trachinian Epialtes, not of Onetes and Corydallus; and it must be supposed that they used all means to learn the truth; and secondly, we know that Epialtes was for this cause banished. I do not deny that Onetes might know the path, even though not a Malian, if he had many times been in that country; but the man who guided them by that path round the mountain was Epialtes, and on him I here fix the guilt.

(speaking for myself I would bet he simply wanted a nice reward because if there was any rumor of a more interesting motivation Herodotus, who loved stories, would have likely recorded it)

There is a certain ambiguity in this passage, as he did not flee his homeland because he feared reprisal from his neighbors, but rather from the Spartans, he was only officially declared outlaw after he fled, and he was killed for some unrelated reason. And it is worth noting in this context that Malis (which had some sort of political form) was officially Medizing,^1 as they paid tribute to Xerxes--however Herodotus records a few examples of men of Trachis proving some assistance to the Greek army. It is obviously an absurd anachronism to portray the Medizing Greek polities as ancient Vichies that rigorously enforce political allegiance to their conqueror, however it is easy to see how in a region that capitulated to the Persians as a whole, Ephialtes' crime might not be seen as so heinous.

This said I do want to sound one note of caution regarding how "foreign" different Greeks saw each other. It is very true that there was no one Greek identity, people, or state, and that the people we call "Greek" were part of many squabbling political entities. However it is highly unlikely that someone from Trachis would see an Athenian as almost as foreign as a Persian, and indeed they would see each other as fellow Greeks. By the early fifth century a sense of "Hellenicity" had already developed and was widely felt, most vividly in pan-Hellenic festivals such as the Olympic Games and institutions such as the Oracle at Delphi. However, what this general feeling of ethnic kinship cannot be read to imply is a general feeling of ethnic solidarity or unity. It does not take much reading of history to see that some of the bitterest conflicts are between groups that see themselves as kindred.

^1 I checked Hansen and Nielson's Inventory of Archaic and Classical Poleis to see if there was any more information regarding Trachis and Malis as a whole, but it is pretty sparse. Perhaps noteworthy is that by the later fifth century BCE the region was heavily under Spartan influence