Yeah,
I had almost the same with Igor! The first time you go into the Necromancer's cave and talk to Igor BE SURE to say (choose the correct conversation option!) to Igor he must WAIT to poison Jonelath and you say him YOU will decide later !!
Or you can tell him to go ahead and poison Jonelath, and then say to Jonelath: hey, have you heard about Igor wanting to poison you? Jonelath will now kill Igor in your absence and he will become your necromancer by default.
