"Vengeance" adds a "moral" overlay to what is essentially the law of the jungle that prevails in organized crime. From the moment Michael returned from Sicily to become head of the family, it was kill or be killed. The other families, taking their cue from Vito's weakness in giving into the drugs business, saw the Corleones as vulnerable, and Michael as an apparently unready heir, and an easy target. Michael knew it, Vito knew it. That doesn't excuse Michael because, in the (later) words of Hyman Roth: "This...is the business...we've chosen."

As for "breaking the peace": Barzini arguably broke the peace by arbitrarily moving in on Corelone territories. And, 'twas he who planned to kill Michael, even though he couldn't have known what Vito and Michael were planning.