I apologize for not being clear. I was referring to the movie (as no history buff knows exactly what happened, and I know far less along those lines than they do).
I only saw the movie once, and it was a while ago, when it came out. But my understanding of the term ambassador is a situation where both sides have agreed to have an agent of one side on the soil of another, under a guarantee of protection.
As I remember the movie, there was no agreement by the Spartans to receive these emissaries, so the guarantee of their safety rested solely in their assumption that Spartans would be too fearful to attack them. They bet wrong, and they paid for it: they were not due any protection.
If I've misremembered, and Leonidas (in movie) did in fact at first guarantee safe-passage of the guests, and then changed his mind--well, that is an act of war, and he got what he asked for (of course, it was coming anyway).
I think the modern movie audience gets that, before the Geneva conventions civilized war
p) in the last century, deceit and treachery often marked how adversaries in a war treated each other. Unlike the genteel and legal wars we have today.
So, it was evil? Sure--wars used to be evil like that.