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.

I believe that, despite his insistence that the killing of Sollozzo and McCluskey was "only business", to him it was absolutely personal.
When he told Kay,"that's my family Kay,that's not me",the Corleone "business" was still an abstract concept. Once it hit home,and almost cost Vito his life, Mike had no problem getting involved.

If you watch his face during the planning of the Sollozzo hit,you can actually see the transition.
McCluskey's murder was absolutely personal, a payback for slapping Mike around in public(sound familiar?

In all fairness,once Vito was almost killed,and Sonny got hit,Mike realized that he was the only remaining defender of the Family.
Having said that,I also feel that Mike actually enjoyed the power and fear that he wielded.
Every slight had to be avenged,but the only solution was murder.

He would never be like Vito.