Speaking of restraint: Michael actually did a volte-face:
When he first steps into Vito's study after the shooting, he finds Sonny making his hit-list. "You gonna kill all those guys?" he asks. "Wait, Sonny..." is his advice. But after the hospital and the broken jaw, he tells Sonny & Co.: "You can't wait..."
Logic was on Michael's side in both cases. Before the hospital, he sensed that, if Vito survived, his chances for negotiating a settlement favorable to the Corleones would be greatly diminished by further bloodshed. But after the hospital, he saw that Sollozzo was determined to kill Vito, and that he had to go, even if it meant killing McCluskey, too. Of course, this could lead to the age-old debate about whether Michael's advice and his volunteering to kill S&M was business or personal--but I'm not going there!