Then why Halsin.
He makes no sense in this context if everyone else in the grove acts so extremely antithetical to his beliefs. It almost makes me believe his defining flaw will be that he never-ever pays attention to his rank and file.
Not everyone in the grove rejects Halsin's beliefs. Rath exists pretty much entirely to inform us that there are druids who abhor Kagha's views. There's also that druid with the bird who is searching silently for Halsin and presumably Nettie since she's going on with her normal duties rather than working on the ritual. Zevlor even points you in her direction which he wouldn't do if Nettie was refusing to heal outsiders. The druid trader also seems concerned for the tieflings and seems there to pass out supplies.
I also don't think everyone working on the ritual is necessarily as extreme as Kagha. Desperation can push people to do uncharacteristic things and we are told this is a very desperate situation.
What really needs to be done is show the desperation more on the side of the druids. Maybe have one talk about how the regional forests have suffered because the druids have had to re-direct their efforts into aiding the tieflings. Have another mention that their magic has been stretched thin trying to accommodate them and that several druids have died defending the grove from monster attacks.
And like I said earlier we need an NPC to direct us toward investigating Kagha. The bird druid who doesn't want to talk seems like a good candidate. if you press the issue maybe she tells the PC that she's heard whispering from inside Kagha's room and is suspicious of those rats that are always following her but don't respond when spoken to.
Also the opportunity to join the Shadow Druids if you're a druid or ranger would be really cool. I have a neutral evil druid concept that'd fit in perfectly with what they are all about.