The rosetta stone for this discussion IMHO is the meeting between Michael and Fredo in Havana. Mike pointedly asks him if he knew Johnny Ola, and Fredo lied to him. Then when they go out for the afternoon, and Fredo is loosened up by a couple of banana daquaris he seems to be remorseful and is having second thoughts about what he did. After Michael confides in him about the upcoming attempt on his life Fredo says some telling things. He says "Why couldn't wehave spenttime like this before?" Before what? Obviously his treachery. Then even more telling "I was mad at you!" Again for what? and finally, the very freudian statement that sometimes he thinks his Mama's jest that he was left on the doorstep by gypsies might be true.
In other words he is not a true brother to Michael.

This scene has guilt written all through it.