Sorry, Cristina, I don't buy the whole weak-and-dumb-Fredo thing. He was a detriment and he needed to be eliminated, or he could have caused harm again in the long run.

And, yes, it haunted Michael for his entire life. It was the one thing that he couldn't live with, but I don't think that he missed Fredo. I think it was because he was so haunted by the fact that, although he hoped to follow in his father's footsteps, he became a completely different man, lacking Vito's warmth. Somehow, he had grown in power without holding onto the love that Vito could inspire.

If you remember his confession in GF3, or his little speech at Don Tomassino's coffin, that was his regret - that he had disappointed his father, that the man that he had become would've broken his father's heart. In his confession, he doesn't say, "I murdered my poor weak and defenseless brother who I loved so much." He says, "I killed my father's son." To me, that is his true remorse, and IMHO, supports my theory that, even though he loved his brother, he realized the necessity of his elimination for the long-run protection of the family.

When he kneels at Don Tomassino's coffin, he doesn't ask how he could've killed his brother. Fredo never enters into it. He asks why he (Don Tommasino) was so loved while he (Michael) was so feared. I also believe that in the scene, he is actually speaking to Vito.