Just throwing in my 2phone cents.
IMO, anything is fair in-game. This is Diplomacy, a game that people would lie, bribe, insult, and "cheat" in order to win. The game was made to encourage players to use every available means in order to win. If a player can make another player act rashly by insulting them, that's quite possibly good play.
That said, out of game is completely different. We are all members of this community and on the forums, we need to respect each other and others opinions or sour the whole community.
So, my solution: everyone needs to take a big chill pill in game and realize that people might not even mean what they say, but are just saying it to get you to do a certain action. If we all realize this, perhaps we can all remember its just a game too.
Side note: I doubt many players on here or webdip would call me an asshole or anything like that, yet I've insulted people intake for just the reason I outlined above. Does it mean I really mean those things? No, definitely not.