Originally posted by Sicilian Babe:
There is no way Michael could've let Tattaglia live. The Tattaglias not only killed Luca, but they had Carlo set up Sonny. Even though it was Barzini who approached Carlo, the killing was in retribution for the Tattaglia son. So Michael had to kill him to avenge his brother's death.
I think the assassination of Sonny was more of a strategic move to either help the Barzini/Tattaglia alliance win the war, and/or force Don Corleone to negotiate, than it was direct retribution for the killing of Tattaglia's son.
In the novel, Puzo writes
.....the enemy was making its plans. They too had analyzed the situation and had come to the conclusion that the only way to stave off complete defeat was to kill Sonny Corleone. They understood the situation better now and felt it was possible to negotiate with the Don, known for his logical reasonableness. They had come to hate Sonny for his bloodthirstiness, which they considered barbaric.
