Ultimately fredo was dumb and got manipulated easily by Roth and Ola. You can blame Mike and Vito for not treating Fredo right (which i have done in the past) but now come to think of it its Fredo that messed up big time.

Even if fredo was planning against Mike he was realy sloppy. The boathouse meeting showed that Fredos motivation was all emotion and no reason.

Fredo was trully a dumbwitted boy who was in the wrong family.