I think Sonny shouldn't have been killed,if he hadn't it could have been good when vito handed control of the family to Michael,I think Sonny wouldn't have been happy and would have spoken out against Vito(like he did during the Sollozo meeting) but Vito would have talked him round,I also think Tessio would have tried to get Sonny on side to whack Michael but then Sonny would have whacked Tessio.what do you guys think?

If Sonny had not been murdered, then Michael's control of the family probably would not have taken place.

Welcome, Kriskaos!
As Vito said near the end, "I never wanted this for you, Michael." Had Sonny lived, Michael would have finished college, probably become a lawyer, and would not have been directly involved in the "olive oil business."

Even if Sonny had lived, Michael still would have killed Sollozzo, gone to Sicily, met, loved, married and lost Appolonia. This loss was Michael's turning point and changed him from forceful to completely ruthless.

I think that he would have returned to the US, become a business major in college, and then taken and expanded the Vegas empire. Sonny would have ruled the East and Michael the West, and poor Fredo would have had nothing.

Wrong. Fredo would have had one brother that wouldn't have killed him.

But if Sonny, as the older brother and "head" of the family, was taking care of Fredo, would he have resented it as much?? Or at all?? If his resentment of Michael stemmed from the fact that he, Fredo, was passed over, if Sonny had lived, would he have ever gotten jealous? And therefore would he ever have betrayed the family? Or was his resentment (look at the way that Sonny spoke to him on Pearl Harbor Day) inevitable?

If Sonny hadn't been killed, he would have been killed a few weeks later.

fredo would have been too scared of sonny to plot against him although in a way he was scared of michael but he was his kid brother so fredo felt insulted but yes sonny would have been assassinated sooner or later his temper would end up getting him killed

Sonnys temper would be exactly why he would also have been passed over for boss of the family,his hot headedness would have destroyed the family now michael he was smart(look at part3 by then he is a billionaire)and vito knew michael was smart and vito could never forgive sonny for bringing michael into the family business.