Put another way, Michael did everything Santino wanted to do, but the difference was he did it without passion. The first time Michael steps up is when Tom is trying to talk Sonny into making some kind of deal with Sollozzo (Tom is constantly telling his "brothers" to make compromises). Tom is not receptive to this but agrees to listen when Michael interrupts and says they are going to kill pop, thets the key for them....after which he volunteers to kill Sollozzo and McCluskey. Tom immediately tells Mike that you just dont go off shooting police captains, and Michael cooly tells Tom that a news story about a dishonest cop getting what he had coming would take away the stigma of his murder, and he goes on to tell Tom that there are news people on the payroll (for someone not interested in the family business, Mike had a lot of information). I dont think SOnny would have thought it through, and I dont think he would have had the patience Mike showed in getting revenge, as witnesses by his thoughtless rush to protect Connie from Carlo.