Hold up, Kagha doesn’t encourage you to slaughter the Tieflings. She orders you to offer to go mercenary for them to get them out of her hair. The genocide here is a player choice. Also, she is actively working to expel the Tieflings — that’s what the Ritual is.
What do you mean "thats what the ritual is"? Expelling the Tieflings is a ritual, wut? She also wants you to kill Zevlor. She doesnt stright up wants you to kill all the refugees but doesnt mind it eaither.
And all for what reason?
Further — the Goblins don’t even care about the Tieflings, Druids, or the Grove. They’re after the gang of Adventurers you help save/let die upon discovering the grove. They state they’re after those people — presumably for stealing, assaulting, or trespassing.
They state several times they want to find the Grove, even Minthara sends you to specifically find it.
Ok, not because of Tieflings but they want to find the Grove and kill all the heretics.
So, its simple — Goodberry isn’t cutting it with all the mouths they have to feed, and what is left is being stolen by the tieflings (break into that larder shack in the Grove as you arrive).
Why is that then not stated by any Druid?
The druids are merely seeking to conserve their way of life, and feel they’re being taken advantage of (which they are) and when their Charismatic leader goes missing and presumed dead, the only person stepping up with -any- direction is Kiddy-killer.
None of that is explained in the game itself. Its only what you basically invented or inferred by yourself as explanation. There is no Druid who actually says "i feel being taken advantage of".