I agree with what Apple says about this matter and I'd like to add something else. I always thought that this subject is pointless whether Sonny would have killed Fredo if he was in Mike's shoes. The reason is, how you answer this question:
Would Sonny have stayed alive long enough to be a successful Don and provoked Hymen Roth or Barzini? I say even if he'd survived that time, he eventually would have gotten himself killed by making some other irrational crazy decision. The whole point was to show us how he wouldn't make a good Don and it doesn't matter what not a good Don would have done to Fredo, because it would have been wrong. Traitor must die.