What you say may be true, but Cristina made a good point, I think.
Sit Fredo down and explain to him that he better be damn careful going forward because if he ever screws up again he's dead, keep a close watch on him at the estate, never let him go anywhere alone, and treat him, in general, like a twelve-year old, and I think all the bases would've been covered and Michael could've let him live.
And in and of itself, the fact that "he was no longer of any use to the family" is not justification for killing him.
First of all, he was still a loved brother and uncle.
And second, even if he did have some "use", by your logic that should not have been enough to save him.
