Congrats to Krlea, who posted a question that I don't believe has ever been asked before! Will wonders never cease?
I'm with everyone else here: Sonny probably would have beat up Fredo but not killed him. Temperament: Sonny was all action, rage, do-it-now, but once the rage passed, he wasn't one to brood. Michael had plenty of rage that he contained, and expressed in an unforgiving nature backed by complex plots that he rationalized as necessary to provide safety to his family and himself.
But Apple raises the most intriguing point: would Fredo ever have betrayed his older brother, since he probably wouldn't have resented Sonny the way he resented Michael? I'd add that Sonny, the more emotional of the three, might--just might--have been more sensitive to Fredo's feelings, and made more concessions to his sensibilities. An older brother can be generous to a younger one who's less competent, but a younger brother like Michael would look down on his older brother and respect him less--which is just what happened.