Real world religions and philosophies exist.
Fictional religions and philosophies also exist, in a different way.
And most religions and philosophies in TTRPGs, put simply, are not good. They are remarkably simple. This is especially the case when it comes to those of the ‘other’ in TTRPGs centred around violence: the orcs over the hill, the insectoid empire, the plant people.
Often, this is because of our old friends racism, orientalism, and exoticisation, painting all ‘non-us’ (i.e. non-white, non-Christian) beliefs as existing in a simple-minded monolith not possessing the intellectual nuance of ‘our’ beliefs.
In such cases, the secondary belief-system is bad for bad reasons.
I’d like to argue against my common fondness for anthropological worldbuilding, which I have spoken of often before.
In particular, in the context of a game, it sometimes might be bad.
Let us take a generic fantasy religion of ‘those people who come over the hill sometimes and kill us all.’
Let us say that their religion has a tenet: ‘killing outsiders is a holy act’. And that’s it. That’s all they believe.
This is enough to drive them to be our major antagonist, though they’re far from subtle.
Now let us add a second tenet: ‘dying in battle is a holy death’. This combination of beliefs is common in various worlds, usually ascribed to orcs or similar. Now suddenly, any human-ish culture will have factions divided between:
Tenet 1 = Tenet 2: We should kill outsiders and die in battle.
Tenet 1 > Tenet 2: We should kill outsiders, but try to live as long as possible so we can kill yet more people before eventually dying in battle.
Tenet 2 > Tenet 1: It does not matter if you succeed in killing any outsiders, so much as the act of dying in battle.
X Tenets: This seems dangerous. I’ve heard good things about the fertility deity over the mountains?
Unfortunately, for the purpose of storytelling, sometimes simple is good. It adds dramatic interest and, as long as it does not end up drawing on real-world stereotypes, can make it easy for the players to understand what the enemy’s motivation is.
But even simple belief systems have problems.
Let’s go back to that simplified religion. ‘Killing outsiders is a holy act’. Debates within that might include:
Is it more holy to go out and kill right now, or to wait and train in order to kill more effectively later?
Who is an outsider? Are those of us who do not conform outsiders?
Which forms of killing are most holy? Does the amount of effort and personal risk matter? Because if not, I’m learning how to use a ranged weapon.
As above, we’re trying to build a world that’s fun for players to navigate. Some players genuinely do enjoy hypothetical theology and the political intrigue that comes with it.
But unfortunately, any time that you suggest there is room for debate and discussion within a people, you also suggest that violence is not the answer. After all, if they can negotiate among themselves, there are many alternatives to violence, theft, and so on.
It’s why it’s often more interesting (if the goal is to allow uncomplicated violence) for the enemy to be utterly non-human (a swarm of literal insects, demons, tree-people) than modelled on actual human cultures. Even if, of course, all three of those arguably have real-world associations that still need be negotiated.
Such non-human peoples can all believe the same thing as each other, but have fundamentally alien essential needs to us that make it beside the point. While all dream-eating butterflies differ on the question of whether their religion obliges them to kill everything now or wait and die wisely later, they could never understand a human being as something needing negotiating with, and thus will continue to eat human dreams.
They might also have such alien intelligences that the things they are concerned about are almost nonsensical to a humanoid intelligence. A bird does not understand the human concept of owning a piece of land to the exclusion of any other lifeform; a human does not understand that the reason for a dispute is the current currents in the air; a collective intelligence might not understand fear-from-isolation.
To one being, a humanoid intelligence might be fundamentally food, in the same way that humans could not truly negotiate with sheep. They might spare a sheep, but it will be on a human’s terms.
And thus they can end up being an existential threat. With a simple belief system.
And one that is, not coincidentally, substantially easier to run as a GM.