Fredo has the opposite weakness as Santino did. He was callow and weak where Sonny was strong and rageful. Sonny would probibly get so angry, he would probibly slap him around as was prevously stated.

I doubt he had the cold pre-disposition to beable to murder his brother. I also doubt Santino was as ruthless as Michael, he wasn't as power hungry as Micahel and therefore wouldn't ascribe to Michael's cold unforgiving attitude. Nor would he necessarily only see one type of punishment would be required for anybody who even remotely betrayed him.