With all due respect, Michael is not only feeling remorse for killing Fredo. He's examining how his life and destiny were altered. Look at how Michael has changed from the beginning of GFI to the end of GFII. He's gone from being a warm-hearted, compassionate, determined young man to a cold blooded, calculating, detached, and untrusting middle-aged man. Everyone Michael loved is either dead (Vito, Mama, Sonny, Fredo, Apollonia) or estranged (Tom, Kay, and his children). Yes, he does recover somewhat in Part III, but he has definitely lost much of his humanity along the way.