Notice how Michael waited until his mother passed -- was it to not shock her, or was it to lull Fredo into a false sense of security?
Either way, Michael killing Fredo was the apex of his evil behavior. From there on out, he spent the rest of his life redeeming himself; the assassinations in GF3 were orchestrated by Vincent and Connie.
Mike didn't want his mother to see another son die. As for redeeming himself in III,don't forget that Mike ordered the killing of the Archbishop,Kenzig,and Lucchese.He remained an unrepentant, vengeance driven sociopath right up until his (in my opinion)well deserved,lonely, and largely unmourned death.
