Everyone is entitled to their opinion of what is fun.
Everyone is entitled to seek out their version of fun in a roleplaying game.
Everyone is NOT entitled to inflict their brand of fun on other players at the table if the consensus is Player X’s variant of fun, isn’t really all that fun.
Look, I get it. You saw some cool Unearthed Arcana or homebrew on Reddit. There are some amazing classes, skills, and feats out there. However, not all of them fit in with the structure of an organized game. One example that readily pops to mind is guns in low fantasy games. Sure black powder rifles could narratively work but chances are aliens didn’t land in your world and introduce lasers when most technology is equivalent to Earth circa 1300 AD.
There are reasons that Adventures League and Pathfinder Society place limits on what players can do. This is to maintain a level of consistency in a game. A player could readily pick up from a game at one Friendly Local Game Store (FLGS) and travel to another FLGS and have a reasonable expectation of how the game is going to run. As a player, you should also be able to go to conventions and other organized play events, maintain those same level expectations, and have a similar experience.
The problem develops when you or someone else approach a game that isn’t run as rigidly. Not all of them are, and for good reason; not everyone wants to “stick to the books.” But if you and your group do want to play everything above board and a new player joins and their idea of fun is trying to assassinate the rest of the players while they are sleeping and run off with their magical items a problem can arise. Or they want to hunt down a laser of Zod and don’t care what it costs the group.
As the game master, you are the defacto leader of the table in this situation. If you don’t know whether the table is okay with this behavior let it organically develop. Maybe this level of intragroup conflict is a dynamic that your players want to explore. However, if it becomes readily apparent that your players do not want to participate in this kind of behavior step in.
While the player’s temperatures may start to rise in this situation you need to stay calm. It’s really tempting in today’s society to overreact. Somewhere during the social media age, we lost the ability to hold a civil discourse. But not you! Politely ask the player to step away from the table with you and talk about their character and motivations. Maybe there is a legitimate reason their character is behaving in this way. Maybe they are just trying to grief.
If the offending player is solely in the game to grief their fellow players politely ask them to try another game. Let them know that your table isn’t for them. I wouldn’t suggest you yell at them to GTFO.
Sometimes it takes a rusty nail in a group dynamic to get things done. They can help motivate some players to role play less and roll the dice in combat more. Not everything in a game can be handled through negotiating and these cantankerous sorts can really help push the party. Maybe Player X will decide to take on this role. There is room in the game for not everyone in the party to be besties. This isn’t the same as being openly hostile.
Chances are you are more likely to run into trolling players online than you are in the real world. Most people that like to cause pain to others like to do so behind a mask of anonymity. There are others though that are brazen enough to do it out in the real world. Kudos to them that they have the intestinal fortitude to not hide behind a screen.
What’s fun for one isn’t always fun for everyone. You make like a role play heavy group while other players enjoy adventures that resemble more of a miniatures combat game than an RPG. That’s their right. But what people don’t have the right to do is spoil the fun for the group. If someone brings hate and discontent to your table counter them with kindness and discourse. Chances are they’ll conform and play along with the rest. If they won’t don’t be afraid to politely ask them to leave.