While these debates over whether the killing of Fredo was justified are very interesting, I think just looking at that question misses the whole picture. Killing your own brother is rather like dropping the atomic bomb on Hiroshima and Nagasaki--even if it is morally justifiable and appropriate, it's still a terrible thing. Unless someone is a total psycho (and as I've said before, I totally disagree that Michael is), even if they felt their brother deserved to be killed they couldn't do it without deeply mixed feelings and deep sadness that the whole situation came about in the first place. It's totally different from, for example, Vito going back to Sicily and killing Don Ciccio to avenge his (Vito's) family. I think even if Michael was convinced he was justified in killing Fredo, it would still be something that would weigh heavily on him the rest of his life, as we saw in Part III (although it would certainly weigh less heavily that if he thought it had been wrong). If FFC wanted to present a less romanticized picture of the mob, he certainly succeeded with this plotline, regardless of how one judges Michael.