Of course, if Sonny would have just shot Carlo when they had the fight on the street, that would have solved a lot of their problems.
If it had not been the toll-booth, Sonny would have been killed some other way. The whole point of Sonny’s character was to show what happened to a mobster who let emotion rule the day. In part II and Rothstein was explaining about Moe Green’s death, he kept saying that he knew that it was only business.
If you got emotional as a mobster, then you would never become an old mobster, as Sonny did not.
In part I when the Don and Michael are in the garden talking about the other families moving against Michael, the Don avowed as to how he knew that Santino would be a bad Don. He was unable to separate the business from the emotional.