Hey guys, in my playthrough I had the same bug at the end, where I couldn't talk to Mordus or kill him after taking a bite out of the black organ for more source. Although I was able to pickpocket the amulet and key, I really wanted him dead, so I reloaded and figured out a (possible) workaround.
After defeating Mordus, you can ask him about Lohar, the deathfog, and possessing the dwarves, but when he offers you the black organ for source, eat it immediately, don't hesitate. Hesitating and eating it at the last possible moment gives a different narration, and you won't have a choice to let him live or die, the dialogue will just end and it'll be bugged (at least for me). Not sure if this happens every time, though. The first time when I got the bug, I hesitated until the end, and when I ate it my character, Fane, was like "I've never felt so much power!" but when you eat it right away, the narrator does all the talking, and you then have the opportunity to let Mordus leave, ask him more questions, or kill him.
