There are, of course, differences between the final hits in the book and in the movie. In the book, though, several of Barzini's capos switch sides immediately after the hit, which opens the possibility that Barzini had been betrayed. So the tactical hit on Barzini might have been a result of Michael's earlier strategic victory.

Of course, I'm sure that the Corleones would still have wanted Barzini dead for certain, as opposed to alive, wounded, and in police custody.