The point is Michael made the CHOICE to kill Sollozzo and McCluskey, and once he actred on that he could never go back to the "legitimate" world.Of course the irony is, as he learned later, the higher up you go the crookeder it gets. I think all those years Michael was trying to protect his family and make THEM legitimate, and at the end he realized he had to redeem himself first. This is why he makes the vow to "sin no more," and turns it all over to Vincent whom he tells "you can never go back."
Of course the price of his redemption is his daughter's blood.