I think that once Vito passed on the deal,the die was cast. As much as Sonny may have been in favor of the deal,he wasn't going to let Sollozzo off the hook. Even if Sonny hadn't tipped his hand, there was just too much money at stake to let Vito stand in the way,so he had to go. . Remember, after the shooting,Sollozzo wanted Tom to calm Sonny down and talk sense to him about the wisdom of getting into the business. Whether or not Sonny was in favor of the deal would be irrelevant since Sonny's temper and impulsiveness were well known in the Mafia world and his first act would be revenge. He was not smart or business savvy enough to understand that it was not personal. That's why Sonny had to go too.As Solozzo said, the Tattaglia's were behind him with all of their people. Of course we find out later that it's Barzini as well. Remember,Solozzo was not a made guy,so no way he could have hit Vito without the OK from above. The Mob was going into the drug business one way or the other,and to avoid a war,they would OK some "pre-emptive strikes". As we see later,after Sonny is killed,Vito makes an "accommodation" with the other Families, and against his counsel, they vote to organize the drug trade.