"Man is the most dangerous animal of all."
Some notes on violence and law on Thule
Quite a few of the "monsters" in the early areas of the Black City ruins are of the human variety - the wandering monsters and hex contents have plenty of opportunities to run into obvious enemies (Bandits and worm-infected Berserkers) and the not-so-obvious ones - Nobles, Traders, Veterans, and NPC parties.
Taking inspiration from the golden age of Pirates, the American Wild West, and the historical Viking period, interactions with other humans in the ruins should be fraught with violence, banditry, and skullduggery. Once outside the "safety" of Trade Town, the city is a free fire zone. Dead men tell no tales, there's no honor among thieves, and you can't trust other pirates. "Take what you can, give nothing back".
Piracy between Vikings during the historical Viking age was common (at least from the historical resources I've perused). Scandinavia was divided into multiple kingdoms, and even within the kingdoms loyalties were split amongst Jarls and lesser lords. Personal loyalties mattered more than national loyalties on the high seas.
This is carried over to the crews on the Island of Thule. Outside of Trade Town, running into a rival group of Vikings could be a dangerous affair - especially if there's some preexisting bad blood. I'm recommending a new (secret) pre-encounter reaction roll - call it a Disposition Check - each time a mannish group is encountered. The disposition check is meant to determine if there's any pre-existing history between members of the two sides that could effect the encounter - a hostile result would indicate pre-existing bad blood, and a positive result would mean pre-existing good relations.
2 - Very bad history, -2 to encounter reaction roll
3-5 - Bad history, -1 to encounter reaction
6-8 - no modifier
9-11 Positive history, +1 to encounter reaction
12 - Very positive, +2 to encounter reaction
Hostile dispositions could mean the existence of a feud back home, a heated rivalry at camp, or something similar. For positive dispositions, perhaps a brother, cousin or relative by marriage is in the other group, or some of the men sailed together previously as comrades-in-arms. I'm going to put together a list of picaresque reasons for there to be bad or good blood by NPCs towards someone in the player's group - it could be a really funny list. The world isn't that big, and lots of these groups have a history, sailed together, etc - this seems like an easy way to add some of that verisimilitude.
Once the disposition is known, the DM can proceed like any other monster encounter, with a formal reaction roll with charisma modifiers to help determine encounter friendliness or hostility, modified by the disposition.
Law in Trade Town - Duels, Mediators, and The Thing
In Trade Town, Bergfinn and his guardsmen (the Bashers) keep the peace as much as feasible. Emotions run hot, and it's common for grievances that started in the ruins to come back to town to be settled. These can result in duels, mediations, and finally - a Thing.
Dueling is common, and the offended parties can agree to duel with wooden weapons to first hit, to first blood, or to the death. (For first-blood duels, the duel takes place on a large white sheet - the first one to drip blood onto the sheet is the loser). Bergfinn's Bashers are glad to oversee duels and watching a duel is a popular entertainment in the camp. From a rule's perspective, the first person to half hit points is 'blooded". Historically, there was something called "island-going" - the duelists would row out to a small island, hold the duel there, and the guy that actually comes back was the winner - that type of 'Thunderdome' could be fun in a game setting, if not over done.
Another common approach is to have a mediator. The aggrieved party picks someone with high status to go to the offender and negotiate a blood money price (weregild). When I publish the details of Trade Town, I'll include a set of example weregild prices - players being players, there'll be plenty of carnage.
For more serious grievances or resolving blood feuds where even a mediator can't help, one of the groups can request a "Thing". The Thing is basically a large open air hearing with a jury. By custom, all the ship captains get a vote on the jury, with Bergfinn acting as the tie-breaker. The Thing shuts down normal camp life for days at a time, as the two parties campaign and employ politics to get the different captains to vote with their side. Lots of potential there for a change of pace, different style of play for the heavy role players in the group.