Fredo was the biggest liability. Along with Paulie. One thing nobody thought of was increasing the Don's security after his no to Sollozzo... Sonny wasn't mature enough to be a Don, they were all put in tough situation while Vito was in the hospital... Michael really stepped up... they also waited too long to get rid of Carlo...he was a boil to their asses the first time he acted out and showed distain towards his wife, especially after the "shut up Connie" scene... he had no respect for the family