Here the thing is though that Melhed has repeatedly lied to Thalmarkin, broken their word and even a treaty.
That's what you say. I don't know that. I know almost nothing about the situation. And if Melhed were to approach me with a story about how Thalmarkin were trying to force them into a vassalage to their "King of Kings", and that they were forced to sign the treaty under duress and threat of destruction, I might very well agree with them and fight on their side. (From what I have seen of the situation OOC, this is pretty close to the truth, from one point of view.) I don't know anything about these supposed Melhed lies. And a treaty signed under threat of destruction ... I may even consider that invalid, so breaking it is no big deal. If Thalmarkin were to then approach me and try to explain all the "lies" that Melhed may have told, I may not believe Thalmarkin. Or I may not care, because I consider the possibility of Thalmarkin trying to vassalize other realms as being a bigger threat.
It's understandable that right now that is ignored by the other realms who are united against Thalmarkin.
It's not that other people may ignore it for right now, they may not even believe that Melhed is lying. Thalmarkin is attempting to subjugate the entire island under their rule under some "King of Kings" plot. That sounds kind of shifty to me. Like, maybe I can't trust them. They might lie to me in order to get me to help them in their plan to subjugate the island. Maybe I'm next? Besides, my own character has other reasons to not trust Thalmarkin that have nothing at all to do with Melhed or the current situation. I didn't like them to begin with, even though the reasons would probably make you scratch your head and say "WTF?!"
Victors would have to destroy their enemy since peace treaties can't be trusted.
Treaties are made to be broken. If they weren't this game would be soooooooo boring. We'd all be living in peace and harmony. Complaining OOC because someone broke a treaty?
So basically, it's fine to be an opportunist but if it is widely accepted as appropriate behaviour by our characters, thus if everyone plays opportunists, trust leaves the game and that is the very origin of the us vs them mentality.
