Well maybe not a direct threat, but Fredo sealed his fate by going against Michael secretly behind his back only to have "something" in it for him. Stupid and selfish, almost resulted the death of the complete Family. Family is family, and Fredo of all people should know best that you stick with blood since he's the only Corleone left as a man.

Michael probably looked at it from a royal emperor point of view, that even your own blood can make assination atempts to take the throne.

But honestly, that is cruel he actually did that to his own brother, but that doesnt make me view michael as an evil character haha