Halsin seems more "worldly" while the rest of grove, or about 90% of them, are introverts and xenophobic. Halsin allowed the Teiflings refuge while several members of the grove were, silently, against it. Since Halsin was/is the Arch Druid there was no questioning this decision even if many were against it. Once he ran off to save the world and got himself captured Kagha took the reins and so the xenophobia was now permitted to be expressed publicly. Now the dislike for outsiders may have always been there, or the shadow druids may have been fanning the xenophobia within the entire grove rather than just Kagha. Of course, if this is the case, then Larian may need to better flesh this out through secondary options such as conversations between characters, notes and other such things. Realistically there doesn't actually have to be concrete facts for people to hate a group of outsiders even if they have nothing to do with the current issue, there just has to be an issue and someone to aim the blame. After all once they force out the Teiflings and enact the ritual they don't have to care about the outside world, so those demonkin gotta go.
A question that could be better explained is how well Halsin managed his grove. I mean going by what we have so far, he's done a shit job of it. He bailed as soon as he heard about the underground fort/temple and he joins our merry little band if we save the grove. There really doesn't seem to be much concern on his part about what happens in the Grove. He seems far more interested in the larger issue of dark magic saturating the land around Moonrise tower. If it turns out he's been focusing more on this big picture issue rather than his Grove that would explain why the xenophobia had become so prevalent.
I wouldn't say the Teiflings are a hivemind, they're a fairly diverse group, it's simply that they aren't a bunch of little shits towards you no matter what you do. Currently, personally, the most satisfying outcome with that grove is killing most of the druids(99.9% of them) then wiping out the goblins. Most definitely not a good guy go but damn are those druids annoying.