Fredo was weak and stupid.

If you watch the film, later on you hear him tell brother Mikey that he (Fredo), "ran into" Johnny Ola in Beverly Hills.

It was he who asked Freddy for some help. If you remember, Fredo told Mike..."they said you were being tough on the negotiations and that if I could help...there was something in it for me....something in it for me!" Mike: Haven't I always taken care of you?"...Fredo:TAKEN CARE OF ME. TAKEN CARE OF ME? I'm your older brother Mikey and I was stepped over!" Mike: That's the way Pop wanted it." Fredo: "Well, it's not the way I wanted it!"

Being the weaker sibling, Fredo always believed he was being stepped on by everyone else. He saw this as a golden opportunity to make a little mark in his own right.

Freddy, because of the way he saw himself, was very gullible. He truly believed that he could do something important. In his glee, he more than likely did not believe he was doing anything wrong, hence, he could not see through his own self-excitment, that his deed was truly part of a "hit" on Michael.

"You're nothing to me now, Fredo. You're not a friend, not a brother."