I am asking this question because I don't think it has been answered by any of the linked questions, and I am on the verge of losing the player in question.
We are playing a text-based game via Roll20. The game engine is D&D 5e and I have been adapting the Mystara campaign setting to run Tales From the Yawning Portal campaign (with many tweaks and additional plot themes). We are presently in the nation of Karameikos, which is based on medieval Eastern Europe.
I will refer to the players in question as Sean and Colin. Sean is the player who seems to resent Colin. Sean plays Matt Mercer's Gunslinger archetype for fighter (we'll call him Sig) and also dabbles in alchemy. He seems to want to be a sort of Rambo style combatant, but also is quite happy with heavy RP and engages quite readily in all manner of RP scenarios. Sig is stretched thin as he has attempted to do a little too much and I have worked with him to reel it in. His background is intended to be a sort of Street Urchin who happens to be a savant with mechanics who has just invented the first gun in Karameikos. Colin is a Battlemaster Fighter (Colt) from the noble background who is something like a Lawful Neutral Paladin - he has honor and a respect for the "peasants" beneath his station, but he does not flinch at dispatching evil creatures nor does he suffer fools. He is patient, but doesn't respond kindly to having his honor questioned.
Event Number 1
Early in the campaign, Colin took exception to the Sean's choice of the Gunslinger class. He brought up a host of issues with all of the minor tweaks I had made for Sean - I allowed Sean to have a nerfed version of a gun from level 1 - which he was not proficient in (the archetype grants proficiency). This was more for RP and has only served to make him weaker. Colin also had issues with our attempts to make the alchemy crafting system more robust by exploring various homebrew options. He felt that Sig was being given too much power (we were only experimenting the the ideas, not trying to instantiate them, but the argument happened anyway). We eventually reached a satisfactory compromise, but the argument took its toll as Colin is a very skilled arguer and is aggressive and persistent when he feels he is in the right. Sean, by contrast, is far more sensitive and looking to relax by playing the game. The intense nature of the discussion was too hostile for him and he took personal offense.
Event Number 2
The group attempted to make nice with some kobolds while Colt protested. He reasoned that kobolds are evil and would either backstab them or come back later to be a plague on the land. The party largely ignored him which led to him starting combat in the middle of the kobold throne room. They (miraculously) defeated ~20 kobolds while surrounded, and then an argument ensued. Sig lambasted Colt for nearly getting everyone killed with his rash decision, while Colt smugly defended his decision saying "We survived, didn't we?" The entire party thought this was great roleplay... Up until Sig openly provoked Colt by repeatedly insulting his honor and nobility. Colt tried to give him ways out of conflict by warning Sig that he was crossing the line. Finally, when Sig would not back down, Colt smashes his shield into Sig until I ruled he had a concussion. Colt barely missed him with his sword before the rest of the party peeled him off and separated the two of them.
Now, I feel that Sig's actions here were directly fueled by Sean's growing resentment from Event #1. I perceive Sean to be having a direct problem with a certain "smugness" that he detects in Colin, not Colt, and cannot find a way to let Colin just be Colin without taking it personally...
Event Number 3
Eventually the party moved on without resolving anything. I threw an encounter or two their way that distracted them enough to force them to work together. Sig has developed an inexplicable problem with the slaying of kobolds (he is Chaotic Neutral, and his background would point towards him being self-centered), and this caused another clash with Colt. They found the nearly vacant kobold town near the throne room where they slaughtered all the kobold combatants. All the kobolds had fled save for those too old to do so, and I conveyed that message pretty clearly. Colt made some threats to the elderly kobolds, though he didn't seem like he would follow up on them. Sig immediately begin antagonizing Colt again, berating him for threatening defenseless kobolds.
Meanwhile, a member of the party who had been exploring elsewhere found a "Beholder" (really a gas spore) and ran to get the rest of the party. Sig was actually so concerned with these petrified kobolds, that he stayed behind to try to console them, leaving the rest of the party to do battle with this "Beholder". Sig didn't speak draconic and could not really communicate with the kobold elders, and so basically twiddled his thumbs during the combat. Due to Sean being a novice player, Sig also got into some trouble (wandering off, getting KO'd for going ahead) that we have discussed and solved.
Following this session, the players discussed (in character) tactics for moving ahead because we had 4 KOs in one session from people going before the tank. This became a huge problem for Sean - he harped again on Colin being controlling (going back to Event #1), and smug (Event #2), and now feels that he can no longer play the game with Colin. Sig essentially went down the same road of insulting Colt's honor again for murdering an entire community of kobolds. Now we've left it off that if Sig doesn't redact his statements... Colt is probably going to execute him at the top of the next session, and I don't feel inclined to stop him.
IMO, Colt is being roleplayed very well and is doing exactly what he would do in this situation. Actually, he has been extremely patient for a noble, and so I think Colin has done an exemplary job of attempting to play fair and cooperate. Sean, on the other hand, seems bent on exacting some sort of retribution for what he feels is a slight against him and his creativity. I don't think he is accurately portraying Sig, as Sig would have grown up in a feudal system where openly railing against the ruling classes would have one branded as a traitor and promptly executed. Sig would have learned that if he truly had an issue with authority, he needs to be silent and wait for an opportunity to destabilize the system (especially since he is a rather intelligent character). But I hope by this point it is as clear to you as it is to me that this isn't about Sig; Sean is using Sig's outbursts to veil a personal attack against Colin.
Now, Sean is actually a very cool and creative person and is the only one in my group I have actually become personal friends with. I would hate to let him go, but he is forcing my hand at present. I have spent upwards of 8 hours coaching both of them in how to communicate more appropriately with each other and Colin has definitely improved. Sean however doesn't seem to be willing to acknowledge what he is really doing. He seems to be triggered by Colin's behavior. I find that this is far too complicated an issue for me to resolve in the course of a D&D campaign... If he is incapable of not taking another person's personality personally (try saying that three times fast), then I don't know what choice I really have.
The Other Three Players
To clarify, the other three have taken a hands-off approach to this. They enjoyed the kobold-slaughter-inspired back and forth between the characters but not the players.
From what I have heard and surmised, it is getting old now. The rest of the players don't want the game derailed by constant bickering. That said, they also seem to understand why Colt has taken the actions he has - his view as a noble is to protect the lands above ground from the threats below ground.
We decided that the real mistake was the party ignoring Colt's protests to befriending the kobolds. Essentially, Colin offered that, had the party attempted to persuade him to hear the kobolds out (whether to case the stronghold for later tactical assault, kill the dragon and drive the kobolds away, etc.), he would have had more of a reason to tag along.
The way it played out RP-wise was that he was literally saying "We shouldn't do this - kobolds are evil creatures. They will lie and deceive to accomplish whatever their goals. We have no way to trust them. This is a mistake!" And then, finally when the kobolds solicited the group to retrieve the white dragon wyrmling, his battle cry was: "$#%! it you naive fools!" and he shield bashed the nearest kobold guard. This is still considered the most epic battle cry, and brought us many laughs.
My questions are: Am I missing something here? Is there a different way I can approach this situation that might work better or actually yield a solution? I really don't want to lose this player, but I can't keep having these arguments surrounding my game.