I agree that Sonny would not have had Fredo killed. He WAS a hothead, but he often regretted the things he did or said when he was angry. Witness how he tried to apologize to Tom for the "Pop had Genco" remark, even though it was true that Tom was not a wartime consigliere. He had a soft spot for those he loved, and I think that he lacked the ruthlessness to kill his own brother.

That being said, I also think that if Sonny were Don, Fredo would NOT have betrayed the family. Most of his anger and envy was aimed at being stepped over so that his younger brother could take over. Aside from the killings in Louie's, what had Michael really done to deserve the role of Don. Fredo and Sonny had been raised in the business. I think that also had a great deal to do with Fredo's anger. He saw Michael as a Johnny-come-lately who was being handed the keys to the executive washroom without having earned them. IMHO, that just added to the jealousy he felt.