Santino's reaction after the Don been shot was right. What else could he do?Negotiate?Wouldn't that be a sign of weakness?Additionally he achieved two things:
1. Earn time for the family to recover and prove that even without the Don there was leadership.

2. Blackmail everyone in NY that attacking his father would cause another war, a war nobody wanted.

So, even though Santino acted spontaneously, he did the right thing. Remember Michael after the hospital scene, when he says that they wanted Vito dead. The war was the right thing to do. Tom who disaggreed was named a "bad wartime consigliere" by Michael