I have to disagree with the initial premise of this thread, namely that killing McCluskey and Sol was something any button coud do. First of all "any button" would have a hard time getting to Solozzo so long as a NYPD Captain was his protector. Second Michael got the opportunity to kill them both when they requested a meeting on the false belief that he was still a "civilian." Michael risked being shot in the restaurant had someone else on Sol's side been seated at a nearby table. He also had to go to Sicily and live in exile for a long time where he learned a great deal from Don Tomassino who had a good lifelong relationship with him. In the process of all this he lost a wife and a brother, and was only allowed to come back when the family was so weak no one considered them a threat. Between what he learned in Sicily and what he learned in the five plus years under his father's wing was enough to allow him to have the street smarts to be a don.