I understand the sympathy and dislike for Michael for killing his own brother... but just think about a few things...

If Michael lets Fredo live... Michael may die. Tom was left as Don so what happens?

Tom as permanent Don?
Fredo comes in and challenges Tom for Donship?
Fredo is recognized as "Don" of Corleone family by Roth and others and becomes that puppet with EVERYONE else pulling his string?
Fredo is whacked and Tom is whacked and Corleone's become extinct?
It gets messy?

After the Fredo's treachery was known and Roth and Frankie were whacked, there was no real threat to Micheal. Micheal could have ordered Fredo to stay somewhere where he could not present any more problem for Micheal. Perhaps a small bar manager where he could bang two cocktail waitress at a time for ever.

Actually the concept of a divided Corleone family with Fredo being recognized as Don by the family enemies is something I had not thought about before, but it is a possibility and one more reason why Michael should have whacked him. Since Fredo said that "hail mary" at the hour of his death, it is possible that he went to that big cocktail lounge in the sky where to this day he may be banging waitresses two at a time and not have to worry about Moe slapping him around.

I've said it before and I'll say it again.

Killing your own brother, your mother's son, your father's son, is just cold. Michael just became a cold hearted selfish bastard. It no longer was about "saving" the Corleone family. It became about Michael, his power and his ego.

Connie had foresight because he really did become a cold hearted bastard.

I think that's exactly what Roth had in mind:

Roth wanted Michael dead to stop him from horning in on his Nevada gaming empire, and from having designs on his Havana holdings. Roth knew that Fredo would be too weak and stupid to run the family effectively with Michael gone. Plus, Fredo had been fatally compromised by cooperating with Roth and Ola. They could have blackmailed him at any time (as in Ola telling Fredo on the phone: "Your brother's not gonna know we talked"---yeah, right, sure).
With Michael dead, a war of succession would have broken out in the Corleone family. Rocco and Neri never would have sat still for being under Fredo's thumb. Tom, notwithstanding his championship loyalty, would have to choose sides. The more they fought, the less focused they'd be on Roth. Roth would use his political influence to isolate and then take over the Corleone hotels in Nevada. Roth's allies, the Rosatos, would inherit New York. Havana would remain with Roth. Fredo would have to take out a life insurance policy.

Actually DC Connie said he was a "LOUSY" cold hearted bastard, so I guess somewhere aloung the way he lost his lice....maybe it was all that oil in his hair.

I don't condemn Michael for ordering Al Neri to murder Fredo; Fredo was a traitor, so he deserved the punishment that a traitor should recieve.

...but they could have made a meal out of the silk in his suit...

"I don't like to see you come out to this clean country in your oily hair -- dressed up in those silk suits - and try to pass yourselves off as decent Americans."

I guess "decent Americans" had permed hair and polyesther suits, like the Dad in the Brady Bunch.

Its really THIS SIMPLE = You kill your brother or your brother kills you.

Which brother are you? Caine or Abel?

Fredo deserved what he got.Thats the rules of the game.Fredo knew the risks and thought that just because Michael was his brother,it excluded him from the rules.

If Michael lets Fredo live then Fredo could be used against him ALWAYS.... (something in it for me on my own)

Another family could take Fredo and give him his own Corleone family and pit him against Michael. Then Michael has to deal with that situation. Michael Corleone family vs. Fredo Corleone family and ultimately Michael would be taken out.

I don't think so.

See, I don't think Fredo was that dim. He didn't want Michael dead, thats for sure. He MIGHT'VE been more caucious and all. Still, he WOULD be an easy target for Michael's enemies, which is what worried Michael.

Michael also might've looked it from ScarFather's POV, because he was at that time, to phrase Coppola, "as mad as Nixon".