Ephialtes was a real historical figure who betrayed the Spartans. Of course, 300 is a fantasy adaptation of history, but it would not be in character for the Spartans to be polite with someone like that. They are famously laconic hardasses who only respect strength. A weak, incapable man would not deserve to fight alongside them.
Fair enough (though I have heard the movie, or rather the original comic, gets some notable cultural details wrong), you just think the main character (or any of the others who heard the exchange) would be smart enough to realize the guy who is now screaming at him in fury MIGHT reveal to the enemy the path that can lead to the 300 soldiers’ downfall. Like, they’re hardasses, but they’re not dumbasses given the thought they put into their strategies, so one would think they would consider, “guy tells us about secret path that could greatly aid our enemy if discovered” and “same guy is now screaming at us in anger” and put two and two together.
Like, I feel if Ephialtes had just been shown with an absolutely furious expression on his face but didn’t actually yell out, and everyone else had already turned their back before seeing his anger displayed, it would fully justify the Spartan soldiers never considering the possibility he would betray them.
But again, I assume this was mostly present in the comic as I’ve heard the movie is mostly faithful to it, so this isn’t even the fault of the movie script or Snyder. Like, when he did majorly change a comic detail in an otherwise faithful adaptation, he altered the source of the massive detonation in Watchmen, which even as someone who loves the comic, I thought the change worked fine for the movie version, but lord did some readers of the comic proceed to nitpick every detail about that change and how it logically wouldn’t work.
And I’m not one to actually let minor details like this affect my opinion of a film’s overall quality. As I said, 300 is great in my book, probably tied with Man of Steel as my second favorite Snyder film (his Dawn of the Dead remake being #1, really solid remake though I do prefer the original). Honestly, I feel people these days do focus too much on such details, and forget that certain popular “critic” personalities online who do so usually make it clear they’re playing a character or doing a bit and this is not how they recommend people watch movies, like CinemaSins in particular has said a few times that their videos are primarily for fun but not how people should watch movies themselves.