At the end of the movie, Michael is literally becoming the godfather to Connie's child while also becoming the Godfather to the Corleone family, sealing his position with blood.

There's a heavy irony in the scene, as Michael stands in church saying that he "renounces Satan and all his works" while a massacre that he ordered progresses in the world outside. It demonstrates the wide chasm between appearance and reality: This churchly dude, renouncing Satan, is actually semi-secretly embracing the world's evil.

This is the scene of Michael's final transformation: He's becoming the thing that he's been trying to be, the Don, the ruler, and it turns out that this is a pretty violent, pretty awful position to hold. He's being baptized, in a way, but it's an evil, anti-spiritual baptism. He's establishing his new identity through murder.

Sure, he's developed some worthy traits—he's cool, competent, and capable of putting business above taking things personally. But those positive traits are put in the service of violence and destruction. Michael is losing his soul, which is, strangely, his triumph.

He's made a strong effort toward becoming damned, becoming the leader of a criminal empire, and he's finally made it. There's something admittedly impressive about him as a villain, since it took so much work.

He's not just following his impulses like a serial killer: He's disciplined. A present-day parallel is Walter White's transformation in the drug kingpin Heisenberg on Breaking Bad.