Interesting question. I dont think the betrayal was the sole reason Mike had Fredo whacked. I think the major factor was the attempted assasination on Mike and Fredos part in the attempt, albeit unknowning (according to Fredo "I didn't know it was going to be a hit). Remember Mike said to Carlo "You have to answer for Santino" Had Mike been the one ambushed at the causeway, I might think that Sonny would have been hotheaded enough to seek revenge, and if Fredo was involved, he might have done him in.