That would violate the rule of writing that you just brought up because he only existed (1) in flashback and (2) as a source of potential conflict between Starbuck and Lee. In my mind, we would have needed to actually see him in the storyline before being killed, not just in flashback. I'd feel kind of gipped if the Final Cylon were Zac. I still think it's Dee. She's either been triggered a long time ago and has known all along, or hasn't been triggered yet. The Four may have been programmed to awake at the Ionian nebula (or in proximity to what Starbuck brought back with her), but the Final Cylon may have been programmed with a different trigger.