This short guide explains how to get Kill Jarvis quest and when you must to complete it.
Guide to Get Kill Jarvis Quest
Turning evil is the specific result of completing the Kill Jarvis quest in the Tahon Pass cult hideout.
You need -15 morality before the cult leader will give you this quest.
The idea is you do that first, and once the quest is unlocked, you finish it LAST.
Do everything else you can first, then take the -10 morality hit for that quest completion at the very end before completing the game.