As I've said before, I do not believe that Michael believed that Fredo was a threat to him.

If he truly believed Fredo was a threat, he never would have allowed him on the compound. (He is allowed to come visit Mama.) Nor would he have given Fredo a pass for however many years Mama ended up living.

No, Michael killed Fredo not because he was a threat, but because Fredo betrayed him (to one degree or another) and Michael, at that time in his life, needed vengeance against anyone who crossed him at any level.

This is also shown in the confession to Lamberto. "He injured me." Michael's conscience would have been much lighter if he really believed that Fredo was a continuing threat.

"A man in my position cannot afford to be made to look ridiculous!"