When following the questline from Zalyn D'Kilneld you lose Tizmak faction over time. In order to regain it you need to follow the the "Tizmak's Advocate" quest line. If you decide to do "Delightfully Evil" to the end first the NPC will disappear and no longer reappear so you can regain your Tizmak faction. At the end of the "Delightfully Evil" quest line you kill the NPC. I think at least after an hour the NPC should respawn so you can finish the other quest line if that's the route you took.