There is no evidence that Carlo acted against Sonny for any motive other than revenge for the beating and humiliation he suffered. We can speculate all we want about what Carlo might have done that put him in a position to be "blackmailed" by Barzini. But the most likely explanation is that the Family Tree "biographer" made a mistake. Period. The "biographer" made another serious error re. Carlo: In the "tree" section, his death is incorrectly listed as 1954. But when you select Connie/Carlo, his death is correctly listed as 1955.