Now stay a while and listen, my friends, for I am about to share the tale of Sir Asmond McConchar.
As I mentioned earlier, I GM'd an Ironclaw campaign last year. It's a fantasy RPG set in a reneissance-era setting called Calabria that's populated by anthropomorphic animals. The guys who made the game put a lot of emphasis on making the setting feel genuine and realistic, so there's a lot of fluff in the gamebook about laws and customs and currency, little stuff that is usually glossed over in RPGs but would actually cause a lot of trouble for ne'er-do-wells travelling around the countryside in any kind of realistic situation.
One of the measures to increase immersion in Ironclaw is that being a member of the nobility is a Gift (equivalent to a Feat in D&D terms), and noble characters have separate judicial privileges and get a bonus every now and then to flaunt their status for diplomacy bonuses. When we started the campaign, only one player rolled a noble character, that being my brother. His character, Sir Asmond, was the younger son of a baron, who had left home to become a knight errant. Aside from the "Noble" Gift, my brother put all the rest of his starting stats into pure combat. Ironclaw is meant to be a balanced system where social interaction is just as important as battle (not that I ended up managing to convey that properly), and Asmond was basically a walking killing machine incapable of anything except killing things, aside from when he used his Noble Interaction ability.
So pretty early in the campaign, egos start to clash. My brother has a really forceful personality IRL, and he played his character as honestly believing he's better than everyone else because of his noble birth. Even though technically his character joined the party last, he made himself into the leader, and nobody really objected because he one-shot-killed the first major villain of the campaign with his greatsword (which he affectionately dubbed "Li'l Bob"). Asmond was also a very big fan of the school of thought where any problem can be solved by killing the right person. Not too many sessions into the campaign, he ended up killing a bunch of people in the middle of the capital city, working on a sidequest. He got off scot-free, because he could prove they were criminals, and in Calabria you need a noble to testify against a noble in court, but the City Council stopped calling the PCs over for quests any more.
After this came the Princess Bride tribute quest, and the players ended up joining the villains. Right afterwards, the players were told to steal something from a nearby village. Turns out that said village was being savaged by bandits. Nearby, there's an abandoned mine that worked as both the village's warehouse and emergency shelter. When the players enter to steal the shit, one of the villagers runs up to them and tries to scare them away with a spear, thinking they're bandits too (which they kinda are). Asmond kills the villager without a second thought. Then the villager's son runs out from one of the tunnels, going: "Nooo! You killed my dad!" And starts banging his fists at Asmond's chest. Asmond kills the kid too, and later defends the act by saying: "I was defending myself from an attacker."
When the players returned back to the capital, Asmond was staying at the embassy of the Duke that his father was a vassal to. He agreed to take one of the other PCs to sleep in his room "as his servant". Asmond kept mocking his "servant" over dinner, so early next morning the guy got up for Asmond, took a washbin, and snuck up to wake Asmond by dumping it on him. However, he flunked his Stealth roll, so Asmond woke up, grabbed his sword from underneath his mattress (which is where he always kept his sword), and tried to kill the other PC in revenge for the attempted prank. He ended up chasing the other guy down the street with a greatsword in hand, dressed in nothing but his underthings. Eventually, the other guy got away, and the guards arrived, so Asmond was like: "I am Asmond McConchar, the son of the Baron of Thanon. My handservant turned out to be an assassin sent to kill me in the night! I demand that you put all your efforts into catching him! I shall deal with the execution myself." And of course, why would the guards disbelieve him? He's a knight, and the other guy is a homeless hobo.
So at that point one of the other PCs was an outlaw because of my brother. In between sessions, I asked him to tone down Asmond a little, and he accused me of "wanting to play his character for him". As I said, my brother has a very forceful personality, while I've always been very meek, so I couldn't convince him to not ruin the campaign. I ended up coming with an in-universe way to rein him down. The next time Asmond started his tomfoolery, the guards came to arrest him, and it turned out the captain of the city guard was a corrupt, anti-nobility guy who wouldn't give a shit about Asmond's noble rights. He sold Asmond over to a criminal league, so they could ransom him over to his father for a huge sum of money. My intention was that the other PCs would take down the criminal league and save Asmond in the progress, thus teaching him a lesson and building up team loyalty. However, Asmond managed to break out in the middle of the group's raid and joined them in taking down the criminal boss.
After the dungeon was done, the group got in a safehouse, and Asmond tried to kill the washbin-guy again. The whole team and a major NPC they'd saved from the criminals together managed to restrain him. He said he was going to have all their asses over this, so the NPC bent over and slit his throat, killing him.
My brother rerolled a character who was originally supposed to be the exact opposite of Asmond, but ended up sliding into being more or less the same character, except not noble. He spent the rest of the campaign grousing over how the other players had "let Asmond die", though he was fairly good-humoured about it.
The other PCs took Asmond's greatsword "Li'l Bob" with them, and carried it around for the rest of the campaign. I had a bunch of NPCs recognise it and go: "Oh my god, that's the sword of Sir Asmond Child-Slayer!" At one point, the PCs managed to intimidate a guy by claiming the sword was possessed by Asmond's spirit.