Another weakness of Michael's was that he let Fredo's apparent weakness and stupidity inure him to the possibility of Fredo's betrayal. Fredo's seeming harmlessness made his treachery all the more dangerous. As Vito said in the novel: always make your enemies believe that you're less powerful than you are.

I've always believed that Fredo was in cahoots with Roth to a far greater extent than he let on. See this post for details:

http://www.gangsterbb.net/threads/ubbthr...true#Post472494