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 is all this nonsense about "the Viper must strike" then? Isn't the part of the grove where the Tieflings are also going to be covered in bramble? Moreover, why are the voicelines for the Tieflings that suggest you are acting on the druids' behalf when you murder them all?