You can wait before fufilling your Dark Urge until the sidequests are done. Part of being a serial killer is choosing the right time

Actually that’s false.
You get no control over the scene. It happens early in the game, you cant avoid unless you don't long rest at all and if you try to leave for ACT 2, the npc will die anyway. It’s just dumb and poorly executed scene, which takes away magic item, because the quest is softlocked.
So, don’t spread misinformation.