If they do turn out to be Cylons, I'm wondering if it's something where they "evolved" to the point of giving up their Cylon nature and truly became as human as possible. No more bodies to resurrect into, no more Cylon physiology, etc.. When they left they programmed the remaining Cylons not to think about them. The other humalon Cylons began to figure out what happened and were jealous of them, prompting them to eventually attack the colonies.
You then get the dichotomy where the Final Five want to help humanity, where the others seek to destroy it. To be Cylon and make the most of yourself means you accept the mortality of your existence and give up the perks of being a humanlon.