Interesting question. While I do feel that the death of Appollonia was a huge blow to Mike,I also think that the ruthlessness die was already cast.
IMHO, the gangster that was within Mike came out after the hospital scene.
Agreed. One thing that would have been different: Michael would not have had to spend so much time and emotional capital convincing Appolonia that he was "legitimate," as he did with Kay. In fact, a big part of the reason he married Kay was to bolster his "legitimate" front. Appolonia would have accepted him for what he was, just as her family in Sicily accepted him even after he told her father that he was "an American hiding from the police of my country." And, she would have fit right in with Michael's family. In the movie, we see Kay only twice with Vito: when Michael dragged her into the photo at Connie's wedding, and at Vito's burial.
I don't think Michael would have been any more, or less, ruthless if Appolonia had survived and came with him to America. Ruthlessness is part of being the Don. Michael couldn't give his own brother a pass because it would have been a sign of weakness that his foes (and even those around him like Rocco and Neri) could have exploited.
