"Paulie sold-out the old man." When Sonny makes that proclamation, it calls for some suspension of disbelief. While Paulie was conveniently sick and you could reasonably infer that he was complicit in Vito's shooting, it would be great to have plausible evidence that points to Paulie's responsibility for the attempt on Vito. Sonny's call and his exchange with Michael make Sonny seem less emotional and more rational with firm evidence of Paulie's complicity in Vito's shooting. You may or may not argue that it makes Sonny almost don-like...but it certainly points to some strong leadership skills with solid intelligence.
I think the fact that this was left uncertain improved the film.
Sonny is emotional and is not rational, so why should we see him any other way? His pinning of blame without evidence is totally in character and foreshadows his fatal flaw.
Also, not knowing for sure it's Paulie adds a level of suspense, since there's the possibility of the traitor still being at large.
