Vito for being too pig-headed to enter into Narcotics with Sollozzo, and Sonny for directly contradicting his father in the meeting giving Sollozzo the impetus to assasinate Vito and get Sonny go into business with him (which was a ludicrous idea knowing how devoted Sonny is to his father)..but these two events set the wheels in motion for ALL that follows.
In the novel, Sonny flat-out admits that "I shot my mouth off," and thus takes responsibility for Vito's shooting. But that's where Solozzo made a big mistake:
Sol, seeing that Sonny was hot for the deal at the meeting, figured that Sonny would still go for it with Vito either dead or incapacitated. But, Sonny could never have made the deal precisely
because he showed himself hot for the deal. If he had gone for the deal, Clem, Tess and probably a lot of the men under them would have thought Sonny was a traitor to his father, and that he might even have welcomed Sol's getting his father out of the way. (
N.B.: The novel says Sonny was anxious "to get out from under his father's thumb," and saw the drugs business as a way to get his own action.) Sonny could never have functioned as acting Don, or permanent Don, with that cloud over his head.
