OK, the following should settle this whole debate. (Ha ha ha . I hope you enjoyed the joke.)

This is something I've written before in another discussion(s), so excuse me for boring you if you've already read it. I believe FFC's intention -- through the theme and tone of the final scenes -- was to show us that Michael was horribly wrong and culpable in killing Fredo. Fredo was not as a jealous plotter in the vein of Macbeth or Marc Antony, but an inconsequential man trying for once to achieve a taste of significance, and learning painfully that he should not have tampered with his destiny of ordinariness.

(1) The ending murders of GF2 have a common thread: all the victims were real or percieved enemies of Michael's who are now either so powerless, afraid, or changed in attitude toward him that it's unnecessary to kill them. Roth is terminally ill and will die in prison. Pentangeli, after seeing how Michael could reach his brother, is so terrified for his family that he'll leave the Corleones alone forever. I've always believed that if these two men had been left to live out their natural lives, they would have been no threat to Michael. I think the same thing has to apply to Fredo in order for his murder to fit thematically with the others.

(2) As Dona pointed out, when Michael has Fredo killed at the end, we are meant to see this as the climactic unspeakable act and the prime illustration that Michael has become the worst evildoer of them all. I think the singular crime of killing one's own blood -- of even considering it -- is meant to be Michael's alone, the crowning tragedy of his criminal life. If I believed Fredo were capable of doing the same thing -- whether he intended it earlier with Roth, or would do it in the future -- then this would dilute the dramatic focus on Michael's downfall.

(3) To those of you who believe that Fredo DID intend to kill Michael and therefore DID have to be eliminated because he would try it again, how did you feel when you first saw the ending of GF2? Didn't you feel sadness? Did you find it troubling or unsettling? In the flashback scene where you saw Fredo once bantering with his brothers, and now forever silenced, did you perhaps find that painful or haunting?

Maybe you did, but you still believe that Michael couldn't afford to be sentimental: it had to be done and Michael had no choice. But aren't those feelings of regret or sadness an indication in themselves that something was "off" about Fredo's death, that it really "didn't have to be done"?