From my point of view, most of you are overthinking all of this. With that kind of criticism even real history doesn't seem to make sense. Here is the story i felt i was told about during my play.
There was a druid grove in a savage but peacefull land. They lived here and made their druidic stuff, helping travelers who needed it but keeping mostly to themselves. Until that balance was broken by goblins and other malicious species, unprecedently organised and driven by a new faith. Doing their stuff and event venturing outside became dangerous, and staying hidden became more important. They could defend themselves but not able to take down an army. But they still offered safe harbour when a large group of weak refugees from Elturel was nearly slaughtered by this new threat, because it was their duty, the rule of the grove. But living with all those strangers now trapped here is not the same as helping the occasionnal fellow passing by, and so tensions arose, as a powerfull but impatient druid voiced the feeling of many : the strangers should be casted out whatever the consequences for them, since their presence incresed the risk for the grove to be found and invaded by an army. Their leader argued that this would be the destruction of the grove in another way, cause its role was to be a shelter in those lands. Leaded by a darker faction of their cult eager to gain another sanctuary in their grasp, and as their true leader vanished, the second took the decision to seal the grove no matter what, and seek for the best option to do it peacefully. Ultimatum, use of mercenaries to protect the refugee or force them off, anything as long as they're gone. Better the grove under Shadow druid rule than no grove at all, and darkness could be the way to survive those dark times. It could have lead to refugees finally succombing to this evil army or escaping them, to an armed conflict inside the grove where the druids would surely have prevailed, but in all outcomes the grove was still there and defended by those thorns. But since plot armored PCs came and defeated singlehandly this army, their bosses, saved the grove leader, and maybe save 2 children, killed the hag, and burnt a red dragon with magic barrels, the grove was saved without having to change to adapt to a newly hostile land. And Halsin could play his 'wise and forgiving good guy who was right all along' act.
I'm trolling a little at the end, but really, all those stories need to be crafted so we decide the outcome/are the solution. So of course other characters of the world make clearly identifiable mistakes and lack some kind of action power. It's an rpg.
But i find this story very believable and consistent. If your home was invaded, would you go outside and try to oppose the invaders or stay with your family in the most hidden and defendable place you can find, and wait for the danger to pass? And how many of the others would you take with you, and how different?
I didn't even need to considere the fact that we're talking about a bunch of hell spawn people to make sense of how this story got where i find it.