This is very interesting. Sollozzo was the one who ordered the hit on The Don's life, right? I think Tom only mentioned that Michael killed Sollozzo for Vito to realize his one son who wanted nothing to do with the crime family finally joined them. After all, he's the one who planned and seeked revenge for the attempted hit on the Godfather, that should mean a lot for Vito. To me, I view the McCluskey-Sollozzo hit as Michael making his bones, sort of like an induction into the CRIME family.