as upset as he was with fredo he screamed to him "you are still my brother" Micheal would not want his last brother to die at the hands of the cuban rebelion. i think that michael needs to hear the truth come from the people who turned their back's on him so when he makes the sentance of death it does not haunt him.. as angry as he is i think he is weak minded and any amount of stress he crumbles...its like all of vitos traits seprated into each son and they each have the other brothers weekness. when he killed calro his death did not haunt him because he got the truth out of him but the death of his brother haunted him till he died.