In 1 Mike kills the heads of the five families to consolidate his power.

Question: could he not have just killed Barzini?

Barzini was the focus of the anti-Corleone movement. If Mike had targeted him only, then the others would surely have fallen into place without futher bloodshed. Even Tattaglia would presumably have crumbled if Barzini had fallen.

NB Obviously he also had to kill Carlo to avenge Sonny's death... possbly also Tattaglia to avenge Luca Brasi. But Cuneo? Strachi? was that really necessary?