Yes, Apple. I'm wanting to know specifically how Fredo could have threatened Michael from exile because I don't see how he could have been. TB simply pointed out that Fredo was a threat. Okay, how was he a threat? As Woltz pointed up above, Mama Corleone could have lived for decades. Unless Michael figured she would die the next day, he was taking quite a chance leaving (so why di he do it?) Fredo alive since any information and relationships with Corleone enemies that Fredo had could have been put in play at any time. But that only works if Fredo had that kind of information which, from exile, he would not have had.

Perhaps as Lilo and PB have posted, Michael couldn't forgive treachery. It might be that simple.