I would say no. While Santino was more hot headed, I got the feeling that he was more about Family than anything. Don't get me wrong, Michael loved his family, but I think he was more willing to do whatever he had to in order to keep his power and position. Maybe Sonny would have given Fredo a beating, but I don't think he would have killed him.

"Neri, take a train to Rome and light a candle for the Archbishop."-Vincent Corleone