Fredo really was incredibly stupid, incompetent,and envious , He felt that he got stepped over,that no one in the Family (including Mike ) had any respect for him.
I don't think Ola would have had to promise him much. I'm thinking a shiny dime and a new hat would have sealed the deal.
Fredo was dumb but I don't think he was that dumb..... It had to be something enticing enough. Because Fredo was frail emotionally, I can't see him deciding to kill his parents son for peanuts (just think of how Michael felt in GF3 when it finally hit him and he was a cold as it gets). It had to be enough to bypass that thought, which also makes me think it was something that Roth would never actually come thru with. Besides, they could just whack Fredo after it done. Once again who's gonna help or care because it's Fredo.
The reason I say Ola was Roth's flunky is because there was no indication that he was apart of the actual mafia power structure and he was referred as "his Sicilian message boy" ,(or something to that effect). That's a flunky, if you comparing him to Corleone or Roth or Pentangelli. I mean even Willie Cicci was disclosed as a actual soldier. I'm not say the guy was a nobody, but in that dynamic, he was Roth's flunky.
