Fredo would never have pulled this shyt with Sonny/Vito, he says as much to Mike. He felt passed over and couldn't stand the fact that his younger brother was telling him what to do.
In a world where he does betray them?
Sonny would just whip his ass, Vito would probably exile him or sth. Agree with the rest, only Mike would end his fukking life like that.
Only things Godfather III got right:
1. Michael's absolute shame at what he did to Fredo.
2. Santino having a wild ass b*stard son.
3. Mike's son being even more estranged from that moblife than Mike was at the start of Part I.
4. Mike dying alone.
The rest of the movie is just....not that great, a lot of good stuff - but not on THAT level.