Several
All those different emotions show on Michael's face within a split second. At that moment Michael came to the relization that the person he had to have killed was his own brother, and that's why he goes over to him and gives him the kiss of death.
Michael knew, at that moment, that he was now going to have to give the order to have Fredo killed.
He only kept Fredo around for two reasons : the first being that he would see if he could extract as much information that he could from Fredo about Roth; and the second being for the sake of his mother.
Good point DC. It is hard not to interpret the kiss as anything but the "kiss of death" but in that sene Michael is an emotioinal wreck. His worst suspicions have been confirmed "I knew it was you," and he has been betrayed by his own blood.
"You broke my heart. You broke my heart." This isn't Michael the manipulator working, it is Michael coming face to face with the the truth which he tries so hard to deny -- namely that the family he thinks he is trying to save has fallen apart.
Among all the chaos of the Castro takeover why would Michael risk his own safety to plead with Fredo to come with him?
