My only though is that although Michael suspected that it was Fredo, once Fredo confirmed Michael's fear that it was him, Michael was unable to control his emotions at that point.
I agree 100%. There are two times in the Epic to watch Michael's eyes very closely. It precursors him absolutely losing control.
The first is during the Superman scene in which Fredo incriminates himself beyond all doubt. Michael's eyes are furious before hiding his face in his hand.
The second is when Kay tells Michael about the abortion. His eyes are flaming red before he snaps and strikes her.
