You do realize the Realms is not Earth, right? It's a fictional fantasy game setting, and can therefore have whatever lore its creators attribute to it.
“Anything goes” does not apply if an idea leads to contradictions.
You can say “I don’t care, this is how things are” – there is no rule that our fictional worlds have to make sense – but then you have to live with the fact that your world, in fact, does not make sense.
---
TLDR: A statement that all members of a group are necessarily evil without exception makes – even by worldbuilder fiat – no sense as long as they are psychologically human enough to make it possible to have them as protagonists in a class of stories which are, by observation, perfectly plausible and not altogether rare in fantasy worlds like the Forgotten Realms (see below). Since almost all races and peoples are psychologically human enough for that, as a rule it makes no sense to make the aforementioned statement about any group.
---
Let’s take our statement “All Sharran initiates are evil” from above as an example. We can apply it to Shadowheart, so it's not *totally* OT
As a sidenote, it’s perfectly fine to use this as a guideline for what to expect from Sharran initiates. If you’ve met 100 of them and they were all murderous assholes, it’s reasonable to expect the next one to be the same. The cracks only appear if you claim this as a universal truth of your world.
Suppose that your earlier statement about Sharran initiation rites were true without exception. Suppose furthermore, that you exist within the world and have limited omniscience in the sense that you can know everything that is at any moment in time, but have no knowledge about past or future. So you could look into every Sharran initiate’s mind and find that they’re all mostly OK with what they did, and none of them were coerced.
That would make the statement that all Sharran initiates are evil true a posteriori. However, First, this would still not allow you to conclude with certainty that the next Sharran initiate that comes into existence will be the same, because inductive reasoning cannot establish universal truths. Second, characters in your fictional world are not omniscient, so they can’t even know this. The temple around the next corner they explore might be different, and even in your omniscience you couldn’t know what the future holds. Under these conditions, it makes no sense to claim “All Sharran initiates are evil” as a universal, exceptionless truth.
In order to be able to do so, this has to be true a priori, a necessary truth of your world. You must be able to derive this from first principles, which for a fictional world, includes the fiat of its worldbuilder. And yes, you can say “This is how things are in my world”. However, that would make it literally impossible for a Sharran initiate who is not evil to exist. And I can easily imagine a plausible case of that for the Forgotten Realms:
Suppose you grow up in a family of fanatical Sharrans. Your mother is a high-ranked initiate. You’ve witnessed those rites and for some reason, your parents haven’t managed to indoctrinate you enough for you to think this is perfectly fine. You hate it. You feel that killing someone for no reason is just wrong and that the world should not be like that, whatever your parents say. But you know it is expected of you to follow in your mother’s footsteps, for the status of your family within the cult depends on it. You’ve also seen what happens to those who refuse, and you know your mother would kill you herself if that happened, and call it an act of penance. So when the day comes, you force yourself to go through with it. And perhaps you’ll gain enough freedom of movement to run away, now that you’re officially an initiate, and perhaps even make up for what you were forced to do. Are you evil? I’m not seeing it.
To claim that all Sharran initiates are necessarily evil would make this story impossible. For people in your world, it would be a paradoxical scenario, one (ironically) only possible in fiction if anywhere. And not only is it a plausible story within the Realms, I’d say it’s not altogether a bad one for a character background, even if it’s a bit cliché.
So. q.e.d. The contradiction. We’re arrived at the statement I gave in the TLDR above, applied to Sharran initiates. And if the members of a group are not psychologically human enough to make sense for the scenario above, as might be the case with low-level devils and demons, they’re – as this example should amply illustrate – a faceless horde. I’m not sure it’s necessarily so (I might explore that later), but it is usually so.
And for that reason, I adamantly refuse to judge Shadowheart just because she's a Sharran initiate, nor Astarion just for being a vampire, or Lae'zel just because she's a Githyanki. To me all of the above is immediately obvious, and I'm quite surprised that I need to elaborate on it like this.