Solozzo was just the front man. When he was killed, as far as the Corleone Family knew, he was the mastermind. However, as we later find out, he had the backing of Barzini and Tattaglia. And they were killed. Why would he need their backing if he could do it all by himself?
"The toe you stepped on yesterday may be attached to the ass you have to kiss today." -Former Mayor of Providence, RI, Vincent "Buddy" Cianci
