When he fled to Sicily, he experiences the thunderbolt when he sees Appolonia. He falls intensely in love with her at first sight. At that time, he is in a search for his roots, and Appolonia, being the good Sicilian wife, is perfect for the man that Michael is becoming. And when she is killed, I think that a large part of him dies.
Perfectly said!
Remember that Michael was only about 26 when he landed in Sicily in 1946. He had spent a good part of his life until then rebelling against his upbringing and his roots. So, at first, I believe he thought it was a temporary sojourn and was expecting Sonny, Tom and his father to bring him home sooner rather than later. He would then resume his former life as much as it was possible to do so.
But after the thunderbolt, I think he went into a new "phase" of his life: Make up for years of rebellion by embracing his Sicilian roots. Or, as Newton said, "for every action there is an equal and opposite reaction." He embraced Sicily through Appolonia: formal courtship, chaperoned visits, old-fashioned wedding, protection from a
gabboleto (Don Tomasso). I think he might have been contemplating staying in Sicily indefinitely--until Sonny got whacked and Appolonia was blown up.
