Despite the fact that Mike told Tom that Fredo was weak and stupid, he still maintained him in a position of authority. As he stated to Tom: "I give you complete power over ... Fredo and his men". He could have jettisoned him at any time and, thus, not put him in a position to threaten the family at that time. Finally, he did exile him from his crime family and that ended his usefullness to any of Mike's enemies. But Mike was a murderer. He murdered and he presided over a group of people who murdered with no thought about its morality. So, fratricide may have been as natural an act to him as ordering a hit on anyone else.
That's being way too harsh on Michael.
If Fredo was genuinely a threat, I don't think the threat would be totally removed simply by exiling him from the family. Fredo would still be resentful and could still have offered his knowledge of the Corleone family to Michael's enemies and conspired in future plots against Michael. He also could have testified against Michael in future prosecutions or government hearings like what we saw in Part II.
And I think it's ridiculous to say that the fact that Michael used violence means that fratricide was "natural" to him. Mafia families operate outside the law. They can't rely on the police and the courts for enforcement because what they are doing is illegal to begin with. While many mafiosi are much more violent than necessary, if the mob never used violence (and, at least sometimes, killing) they would have no power or credibility whatsoever. What reason, for example, would anyone who owed them money have to pay them, since they would have nothing to fear if they didn't pay? When Michael learned of the plot against him at the end of Part I, what was he supposed to do other than what he did? Send the heads of the other families to bed without dessert? I don't think the fact he was a Mafia don means that he had absolutely no feelings for anyone, even close family members. Tom and Vito were also members of organized crime and they both broke down and cried after Sonny's death.
