I have always felt that Michael had one regret - the murder of Fredo. I believe that he truly felt that that was the one sin, of all the many that he had committed, that he could never be be forgiven for. I'm sure that he felt that the loss of Mary, a loss that no parent should ever suffer, was his punishment for that sin. He lost his child, just like he murdered his mother's son.

Although Vito lost Sonny, it was in the course of business. Mary was a "civilian". I know that John Gotti lost a child when he was hit by a neighbor's car (said neighbor did disappear) and have wondered if that was FFC's inspiration for Mary's death. It's interesting to watch GFII and see Mary as a child, and know that she will live such a short life.