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.
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. Additionally, they’re interested in the Nautiloid. If you follow Sazza to speak with the Drow — they say as much, clearly not knowing those adventurers have left.
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). 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.