First, a little history to this group:
I had joined my brother's TTRPG group and previously played a 6 month campaign using the Harnmaster system as a player. When our DM and host was in a slump for ideas, I offered to DM 5th edition and run them through the new Ghosts of Saltmarsh book that I had just aqcuired.
I was starting them at lvl 1, just to take things slow since I had very little player experience with 5th edition, or any dnd for that matter.
Our group consisted of a Monk, rogue, Cleric, two fighters, a bard, and a sorcerer. I asked them after they made characters, who was from the town of Saltmarsh and who was from out of town. 2 opted for being residents. The monk, and the star of our story, the rogue. We took a long while to get everyone introduced, starting with a bit of roleplay and introduction to the town.
Our monk worked as a bouncer for the seedy pub in town. Our rogue was a regular at this pub, and thus had a previous history with this monk. The rogue was a sea elf, and had a party trick of sticking his head in a bucket of ale, and breathing it to get drunk. He would then be escorted back home by the monk. Which made the monk his best friend.
The party got together when the sorcerer saw a flyer for someone to take care of a "Haunted Mansion" about 4 miles out of town. She went out and gathered people to go and investigate.
Upon arriving to the decades abandoned house, they were all sceptical as to the legitimacy of this being an actual ghost problem, and figured it was actually a group of pranksters or bandits holed up and using magic. My players were all experienced DnD players, and had some pretty good meta knowlege.
They went inside and began to investigate. Finding nothing more than several small trinkets hidden away and rotted wood.
However, our rogue took extreme interest in the brass doorknobs i described, and began to try and use his lockpicks to slowly remove the doorknobs from the doors. He was going for the "loot anything of value" approach I guess. Nothing i haven't seen a rogue do before.
During their looting of the first section, the rogue and monk went down into a small wine cellar to investigate. The monk was immediately attacked by a swarm of spiders.
He took a total of 3 damage in the first round. He had a max hp of 8. Our rogue then took out a flask of oil and proceeded to pour it onto the monk. We were all thinking he did it to try and wash the spiders off of him. Boy were we wrong. The next round, the spiders missed their attacks, and the monk did a little damage to them. The our rogue lit a match, and attempted to light the oil on fire. Everyone at the table immediately began to try and stop the player from doing something so reckless.
For those of who who don't use oil flasks often, anyone set on fire by the oil taks 5 fire damage every turn they are on fire. Our monk had 5hp left, and the swarm had about 8 left (which they didn't know)
The player began to argue that "It's what my character would do! It'll kill the spiders!"
"Yeah, and me!" said the monk player back, explaining that he only had 5hp left.
"Don't worry, we have a cleric who can heal you" The rogue said trying to defend his actions.
"The cleric that is on the other side of the house?" the monk said pointing to the map, showing that the cleric was indeed at the other end of the house, and on the 2nd floor, while they were in a basement level room. "I'll die before he gets here."
"Only if you fail your death saving throws!"
This is when I stepped in, trying to get this resolved. "Look, you can make a ranged attack roll to throw the match into the oil. If you make it, then ill roll to see if the match lights the oil". Probably not the best thing to say. I was still new and wanted to be fair to all people.
Our rogue agreed and made his roll. Nat 1. "The match snuffed out midair and landed outside of the oil" The rogue was reasonable and acknowleged that it was because of his roll.
Next round of combat, and the spiders missed their attacks again. The monk did 4 damage to the spiders. The rogue then, in his infinite Chaotic Neurtral wisdom, tried the match again. I didn't give in this time and informed him that intentially killing another player, after being asked not to, would result in him being asked to leave the session.
The rogue, in a huff, finally agreed and went down to kill the spiders. They succeeded and then climbed out of the cellar. They found the cleric who started to top off the monk's HP.
The party continued to search the house, finding a couple of trapped items, enchanted with magic mouth. Everyone succeeded on their saves and were able to continue down into the basement of this cliffside mannor.
The next session and the party found an underground area, which lead out to the sea, overrun with smugglers. They quickly dispatched the first group they came across.
The party had a good plan of saving the loot sorting until after they clear everything out. They would instead drop everything in a huge pile, and go over it later.
They found a room that was marked as dangerous and that no one should enter. Being the adventurers they were, they ignored the sign and went in. Immediatly they were jumped by skeletons that were covered in an unkown alloy that overcame their bludgeoning vulnerability.
The rogue, who insisted on being the first in the room, had a brilliant plan to deal with this situation. Run away.
He was top of the initiative and spend his turn running out of the room, and hiding behind a barrel about 30ft back. Meanwhile, all the melee fighters ran in and began to clean house. They mopped up the skeletons, and even defeated a special skeleton that made an appearence halfway through the fight.
Between those two combats, and a lot of out of character discussions, we had to call game there.
We play every other week, due to scheduling with the back room we rent from a local church to play in. In this time, every player accept the monk came to me privately and voiced their concerns with the rogue.
Turns out, before i joined this group, the rogue player had a very Player vs. DM mindset, and had sabotaged the party numerous times in other games just to try and spite the DM. In the Harnmaster game i played with this group, the player's characters kept dying before i could see how they played.
Hearing all this, ontop of what i had witnessed over the last two sessions, i asked that the rogue player not to attend game while i am running. The player was upset and began messaging other players asking what he did wrong, before uninviting himself to all future games.
The players had no problem with him as a person, but as a player, they were just too Chaotic Neutral in real life.
I am still dming this group 6 months later, and my group is approaching lvl 7. We have had no player incidents like that since, and everyone is helping me grow as a Dm.