Does your game allow PvP combat actions? If not, loot sharing should be mandatory because it is, inherently, a PvP action. The normal method of resolving such a conflict would be to beat the shit out of the offender until they come around, but if that's not allowed, what in-game resolution is there? As such, an out-of-game resolution is needed, and that is compulsory loot division.
This is why I don't like most dnd dms. Ruling out betrayal because it's 'a coop game' is just as crazy as forcing every session into being a 3 hour combat encounter. If I wanted to play a coop loot dungeon crawler there's about a million video games that automate the dice rolls and loot sharing, lettimg me focus on the action and the loot.
Tabletop's role is to promote emergent story telling, not a more cumbersome way to handle combat and loot. If all a DM can do is stick to their contrived storyline and throw combat at you without facilitating emergent story telling, they're just a worse version of a video game. I'll go play Baldurs Gate instead.
As with everything - discuss your game before you start playing it so that everyone is comfortable with the style of game you're playing. Acrimony stems from people believing they're playing different styles of game.
If you want to play a character who steals from the group or otherwise acts to the detriment of the group as part of their characterization - then make sure the other players are going to be happy with, and enjoy the style of game you're bringing to the table.
If you're designing a character who steals from the group or keeps loot for themselves, you have to ask yourself... are you making the kind of character that the other characters would want to travel with? If your character is just an asshole all the time, why would they not just... part ways with your character? In real life, would you hang out with someone who kept stealing your wallet or refusing to pay their share of the bills in restaurants or pubs? "I'm just roleplaying my character" yeah, well they're roleplaying their characters too... They're not all putzes, and if spott a raw deal you gotta be prepared for them to ditch your parasitic ass.
If you're relying on them keeping you around simply because it's a game and "it's not fair to exclude your character and it wouldn't be fun if they did that", then you have to be willing to make concessions too (like being open to a full loot share), because it's also not fun to have another character constantly taking all the good stuff for themselves.
Ask yourself if your fun is at the detriment of everyone else's fun. If you're making the game less fun for everyone else on the regular... are they going to want you in the game at all?
DnD adventuring parties are, at their core, a group of people who get along, working together, for their common benefit. If you are designing a character who "doesn't play well with others" it's important to understand that your character's personality is pretty antithetical to the game's core expectations.
Definitely, if you're considering contentious characters like this, also consider making a character with the capacity for personal growth. Maybe you start out solitary and paranoid, or greedy and secretive... but you grow to trust and work with these other people in time. Maybe they learn to be less selfish and make sacrifices for the group. Once these people have saved your life a bunch of times, maybe they realize things are more important than money. Character growth makes for a good story, after all, and that moment where your character makes that leap can make for a great story moment.. just... don't hold onto that growth for too long, because the character might be annoying before that realization.
People aren't rigid in their personalities. Yes, a lot of RPGs make you write down some traits on your character sheet, but if you view them as hard and fast rules that never change for an entire campaign, your characters will feel one-dimensional and shallow.
@DoodlePoodle has an excellent comic about this, that illustrates (hah) the point very effectively.
Always remember that the other player characters have agency, and opinions. If your character doesn't play well with others, those others have no obligation to play with them.
My current rogue started out sharing loot fairly but it's become a running joke that she stashes so much she jingles when she walks. To be fair, half of the party actively don't want money and she'll share when asked (if she can't get the five-finger discount on whatever the party's trying to acquire) so essentially she's acting as the party bank.
I play an arcane trickster rogue who sometimes goes off on her own little heists to make some money and she doesn't always share the rewards of that.
Also if she really put her butt in danger to get to a secret chest she might take a lil finder fee before anyone has a chance to look.
Sharing loot is ideal and generally I do so but I think it can be fun flavor to be a little greedy. I'm also super ready to be caught ic and deal with the ic consequences of her actions.
Also she's very loose with her money and buys a lot of gifts and carousing so it sorta evens out
Remember. I said racing to investigate. Meaning its shared loot, not a single mission. If you are the only one risking your ass, I wouldnt be against you keeping it.
In the end it boils down to trust. The party will (most likely) regularly expose themselves to mortal danger. They need to be able to trust each other with their lives. If a character habitually cheats them out of loot they might find it hard to trust that character. So why would they travel with this person?
This doesn't rule out playing an untrustworthy character but it makes it harder to justify their presence, especially over long periods of time. A dodgy thief might be needed for one quest but why are they kept around afterwards? Inquiring minds want to know.
It's a sliding scale. Your arcane trickster doesn't sound like she's super far down the scale so she's probably good even if the rest of the party notices. Or maybe she never even hid it in the first place. That also works.
It kinda limits the spectrum of playable characters though right? The others can notice that that's going on and either go PvP or kick the person out of the party. It can actually be a cool character arc to teach the character to share loot.
Of the player themselves aren't able to learn that though... do the same as above but irl?
Someone that says : my character wouldnt be sharing gold.
Then I say : but who made that character hmm ?
You have imagination. Use it to justify not being an ass to your party.
That's his whole point, you just shoehorned a third of the alignment system out of your sessions with that one move. If that's what you and your players want then that's fine, but it's certainly not better like you seem to think.
It is entirely reasonable to limit the spectrum of playable characters to those who have some motivation to play the game with the other players. Stealing loot will reasonably upset the other players (not just the characters), by reducing the ways in which they can play the game. So no, it's not just creative freedom, it's being a dick. It's choosing to be a dick. GMs telling you not to do that aren't just limiting your creative freedom, they're telling you to stop being a dick or they'll stop playing with you.
You're probably not going to be telling your good party that you're evil and just using them, so anything to help blend in and achieve your goals is a good thing. I constantly use my Chaotic Evil Cleric of Cyric this way, and deception is part of the game plan. I can pretend to be good, if only to better serve my evil god. Now if the thing in question just happens to be something I need for my evil goals, but I can't just tell them, I could simply wait until we rest and steal it while I'm on watch.
I could make it work. They need to not be suspicious, so be generous with them. They need to eat and aleep, so they need some money. Same with their inventory care.
And why do you need so much money ?
Plus at some point, there is this exchange that will take place :
Player : my character has no reason to share/be with the party
Good DM : great. Now go back to character creation and make a character that will work at a minimum with the party.
You can’t beat your dogs and expect them to be useful. True evil is teaching them to do what you need, and then letting them sacrifice themselves for your benefit.