I'll add my two cents. In The Godfather (1972) After Fredo took Moe Greene's side when Michael confronted him about casino and hotel buying out Michael warned Fredo never to take sides against the family:
Michael: Freddie, you're my older brother. I love you. But don't ever take sides with anybody against the Family again. Ever.
Fredo didn't listen to it. Ignored the warn and colluded with Johnny Ola and Hyman Roth, which almost ended by Michael's death.
Even though Fredo didn't want Michael to be killed he probably knew that it was going to be a kidnapping. According to original screenplay by Mario Puzo and Francis Ford Coppola:
Michael: I want you to reach Fredo. I know he's scared, but have one of our people reach him. Assure him that there will be no reprisals. Tell him that I know Roth misled him.
Hagen: My information is that Fredo thought it was a kidnapping. Roth assured him nothing would happen to you.
Allowing "kidnapping" was not so innocent action. Fredo was OK with that.
Taken from answer of @paul: What exactly did Fredo do to betray Michael?What exactly did Fredo do to betray Michael?
At this moment Michael was precautious cold-blooded criminal that protected himself and his family.
It was only in The Godfather: Part III many years later when Michael Corleone probably regretted killing his own brother. When speaking to Cardinal Lamberto:
Cardinal: Go on, my son.
Michael: I killed -- I ordered the death of my brother. He injured me. I killed my mother’s son. I killed my father's son. (he breaks down in tears)
Video fragment: The Godfather: Part 3 (7/10) [youtube]
Also when Michael had a stroke he was yelling for Fredo, probably crying out for forgiveness.
Video fragment: The Godfather - Never Let Anyone Know What You're Thinking [youtube]
