This topic brought me back to my attention while I was browsing the threads and how I recently watched GFIII (2 times actually!). I think the most appropriate answer was probably Kay. I remembered right after he said that he betrayed his wife, he said he "betrayed himself." I don't know if the car explosion with Appolonia has anything to do with him betraying himself or his "wife" - so yes, I think it's Kay.

Even though GFIII was kind of uh, corny? (in some ways, yes - but I thought it can be nicely classified as a "blockbuster" - it was quite entertaining if you ask me!) and the scenes between Michael and Kay sort of didn't "fit" into the script, but I found them the most enjoyable. In Sicily, he asks for Kay's forgiveness, which goes back to him betraying Kay and not being devoted to her as a husband and father to their children because in GFII he was so drawn into power and the family business he didn't even really find time to spend with his family. And of course how he said that the family business would be legit. by 5 years - and it's already 1979!