Seems to me like he is asking people to tolerate a behaviour he would not tolerate from others. IMO he should meet you half wayhalfway instead of rejecting your arguments, but he's not.
So trying to talk with him doesn't work and it sounds to me like it does not work because this player is being very childish, transforming his group's valid concerns into something they are not, iei.e. personal attacks.
I personally can be very firm while also being very diplomatic. Honestly, someone who would twist my arguments into personal attacks and try to turn himself into a martyremartyr would get an earful from me about the fact that we are discussing valid concerns and the fact he's acting like a ... let's say a child.
It would be a very serious discussion about how I do not appreciate his being childish, and if he keeps proving to me than he cannot be civil and respectful during our exchanges, I would totally lose any kind of interest I have in interacting with him.
When a problem player does not want to even start considering hethat his behaviour might be problematic, when diplomacy fails, there are not many options left: endure, leave or kick him out ? Different people/groups have different tresholdsthresholds for that, but that's an immutable fact of life: once you hit the "point of no return", then your options are very limited.
When one of my players useuses this "excuse" without further explanation, I probe them further for an explanation, for deeper insight in hisinto their character's psyche.
A simple way to do it is to ask him, "But why would your character be/think/act this way ?". Ask questions but do not "say stuff about his character" like "He would not be smart/dumb enough to do that", since that would be crossing a line where you start telling him how he should play his character.
Challenge him on his excuses. Like in the instance of the barbarian deciding to invade a whole outpost by himself, when he clearly has no way to win and will clearly end up "badly" for his character, I would have asked him, "Is your barbarian smart or tactical enough to realize he has no chances ? Or does he have a death wish ?".
... whichwhich brings me to my next point.
Never forget that you are a group of people. Usually, permissions given to a player are implicitly given to other players ... else, you have an unfair group and this will lead to frictions/frustrations for some.
So if the logic of "my guy would do that, there's nothing you can do about it" is valid for him, it is also valid for YOUyou.
EG.For instance, I once had to deal with a problem player who was a bit similar to yours: strong headed-headed, constantly shifting arguments to turn a debate about the game into something personal between players, he. He was also passive aggressive-aggressive in how he showed his discontent at the table and was very stubborn that everything he did would be possible (even thothough that never was our argument,argument; we were very clear that possible does not mean believable or desirable).
His favorite argument was, you guessed it, the "my guy would do that, it totally makes sense" line that he would give to us constantly. Frustrations were building to the point we decided we needed to have a talk about it ... which did not work. Basically all we got from him was "you guys want to control my character", when what we really were trying to make him realize was that "Our characters DID TRYdid try to influence him, in game-game, which is something our characters would do and makes total sense ... but since it did not work we now have to have this discussion out of character".
The next week we had a full table (surprisingly), but the problem player kept being problematic. He did more ingamein-game shenanigans which impacted the whole group and derailed the game, once again, when he actually tried to sneak into the King's Chamber during the night we were spending at the Castle to try and steal the King's spellbook. He got caught, and thrown in prison, and our group had to undergo a "trial". We succeeded in convincing the King that the thief was acting alone, and the GM salvaged the situation by having the King "punish us" by sending us on a quest.
But here's when the problematic "my guy syndrome" also became a solution: When the King asked us if we needed the thief for this quest, my character acted according to his own beliefs and personality and said, "No, let him rot in jail and hopefully let this be a lesson for him for when we get back". I have to specify we knew the quest would be done in less than 1 session, so I thought this was a brilliant way to have our problem player get a taste of his own medicine.
When he got frustrated at the table saying that it would be very boring for him to just watch us play for 3 hours and that I was being a bitch, I calmly told him about "those many times where we had to watch him for more than an hour because of his PC's shenanigans and the fact that if my action would cause 3-4 hours of boredom for him, his actions in the past cause at least 10 hours of boredom for me ANDand all others around the table, minimum". I was polite but firm, and when he kept complaining I told him, "that's enough, I don't want to have this debate with you, especially if you are gonna be childish about it". And that was it. He sulked,sulked; we played.
But I believe that made him realize that we had just as muchmany ways to influence the game and his character thanas he did, and that we just had chosen not to do so before, but that we would start enforcing consequences for his character's dumb moves in character.
TL;DR: your characters do not have to rescue the barbarian the next time he derails a mission. Maybe your group will start thinking they are better off without him ... and that's totally legit. It is in characters' reactions to a fellow comrade's actions and is totally legit and even uses the main argument of the problem player against him, in game-game.
