I wonder if Roth would still have been as dangerous to Mike. He would (presumably) have gone to prison,he wouldn't have Johnny Ola ,and any potential rivals might prefer doing business with the new generation. But that's just speculation.

As far as Fredo,he really was not respected by anyone in the Mob,and I think that as untrustworthy as he was,he would never pose a threat to Mike again. He was completely cut off from the family,so he couldn't get any info,he couldn't get near Mike to kill him out of anger,and no enemy of Mike's would trust Fredo because they might be afraid that he would rat them out to get back in Mike's good graces. In short,angry or not,as far as revenge,Fredo was a sputtering, incompetent fool who had lost his brother forever.

I can almost justify all of the other killing,but Fredo's was the one that really showed Mike's true nature and made me despise him.