...When Sonny dispatched him to Vegas, Fredo was proud that he was going to "learn the casino business."...
True, and according to Moe Green, all Fredo learned was how to do coctail waitresses two at a time.
So the 'casino business' proved to be yet another line of work that Fredo was incapable of handling. Mike ended up giving him about as much responsibility as he could handle; there was virtually no way to foresee how discontent and disloyal he would become, at Roth & Ola's prodding.
On a side note - it still is amazing to me how much slack Fredo gets on the Board simply because of his 'weakness and stupidity'. The guy is given numerous chances by BOTH his brothers to make good - and messes up time and time again.
Give him something so he could "believe he was more important to the family than he really was..."??? Man...the guy was lucky he wasn't mopping floors.