Not to mention, it was a war. Did you expect Sonny to be a pacifist during a gangwar? Vito said Tattagia could never have outfought Santino, which means to me that Vito approved of Sonny's aggressiveness. Santino was through it before (as he reminded Michael in the deleted scene I love the most).

Santino was the acting-Don while Vito was out of commission. I think Santino was justified in offing Bruno, as well as all the button men out there doing what they had to do. Sonny was a General, and like Bugsy Siegel once allegedly said, "we only kill each other."

Did you expect Sonny not to do anything during this war?