So - the Absolute's only interested in the death of the Druids - not the tieflings. You can see this in the reports they write up about how to subvert the grove given that it has proven problematic in the past.
A compromise would be to be tasked with executing the Druids or the leaders of the Druid grove with the Shadow Druids and negotiating safe passage for the Tieflings.
The Absolute's forces go back on their word and ambush the Tieflings in Act 2, same as always and some still flee to Last Light same as before.
Minthara - "We should have shadow druid agents within the grove, meet up with them and help them slaughter the leaders - in exchange we will arrange for safe passage for these Tiefling Refugees."
The Druids die, but the Tieflings are allowed to leave - the absolute forces take over the grove, kill or capture and tadpole the shadow druids as well and search the grove for the weapon (to no avail).
It's still evil but not ridiculously unbelievable levels of evil - the Druids are on the verge of committing genocide anyway so it's excusable.
