The old Archdruid when you meet him explain that the circle lost his way. Probably it was a process that was started even before the Refugee arrival. That Circle has also a particular story on the past. If you examinate the slates you can see the place had a very problematic past. I assume what Khaga did was slowly convincing the other druids and bring them to her side. Also not all extremists druids are Shadow Druid. Consider that Sylvanus is not a good oriented God is a Neutral one. You can meet Sylvanus druids leaning toward being good like ones leaning toward evil. In a circle there are various individuals what it matters is what they follow despite if they are good persons or not.
They are seen as Innatural because fiends ((Demon and Devil)) are not native from the material plane Tieflings having Devil or Demonic blood have an heritage that comes from the lower planes ((Abyss and The nine Hells)) so are often seen as creature with Hellish or Abyssal blood and many assume they are evil. The story also take place after Elturel ((one of the cities close to Baldur's gate)) was actually taken from the material plane and dragged in to the Avernums ((that is one of the layers of the Nine Hells)) this also explain further how is bad the situation for Tieflings. People are superstitious.
