I think I know what FrankieFiveAngels is referring to. At an early point in the movie, Michael tells Mary, "I would burn in hell to keep you safe." Much later, when praying beside the body of Don Tommasino, Michael implores, "I swear, on the lives of my children: Give me a chance to redeem myself, and I will sin no more."

This is an apparent contradiction. If he is willing to "burn in hell," it sounds like Michael would commit any grievous sin, even an act deserving of hell's punishment, for the sake of his children. That is, he would do whatever it takes -- good or evil -- to protect them. Yet he later vows to sin no more. Did he have a change of heart? Does he want to have it both ways? Is he just spouting empty words and figures of speech?

Looking at it in a Biblical way, Michael has the audacity to make a bargain with God: Give me this chance to redeem myself. If I don't use this chance to live a sinless life, then the lives of my children are in your hands. God obliges. He sends Vincent with the dire news of a slew of enemies he's uncovered. What does Michael do with this "chance"? He implicitly gives Vincent the order to kill each enemy, thinking that this one-step removal from the crime leaves him innocent. We know and God knows that it does not work that way. God exacts his payment by taking Mary.

So Frankie, I don't know if this is exactly the interpretation you had in mind; but I guess it's one way of looking at it. Mary's death was divine retribution for Michael's sins, particularly for his continuing to sin.