I think that believing that Sonny would eventually make a deal was the big flaw in Sollozzo's thinking. Because even if the hotheaded Sonny had been able to play it cool (which even Michael himself wouldn't have done, after all he was the one who shot the Turk), Sonny would -- in true Sicilian fashion -- have sought revenge for his father's death, sooner or later. Deal or no deal. I believe that it was Barzini all along, as the Godfather said. Barzini must have advised Sollozzo to go after Don Corleone, knowing that eventually Sollozzo would be expendable. The Turk was an outsider anyway. Arguably, Barzini may have planned to have Solozzo killed eventually anyway, or have him move back to the Mediterranean to keep an eye on that end of the drug trade.