They all did, that was noticeable when Michael said he'd kill Sollozzo and McCluskey and they all laughed at him. That part always seemed strange to me when Sonny remarked about him being a nice college boy, like he forgot that Michael was in the Marines and undoubtedly had to kill someone before. It did seem as though Sonny believed him and realized he was serious until Clemenza started laughing but I also think Sonny really had alot of respect for Michael after he planned out how to be able to divert some of the heat from killing a cop and was seriously going through with it.