Originally posted by Turnbull:
Fredo was a danger to Michael and his family before Michael "forgave" him, and remained a danger afterward. That violent outburst in the boathouse--"I was stepped over...it ain't the way I wanted it...send Fredo to do this, send Fredo to do that..."--showed burning, dangerous resentment of Michael that would only have gotten worse, and more dangerous to Michael, over time.
Turnbull makes a very good point.
If I was in Micheals place, I would've killed Fredo as well. Plus I happen to think Micheal was right to have Fredo killed.