Interesting topic, Turnbull. I'm surprised that you have let yourself delve into the third film given your dislike for it!

My opinion is that Michael was trying to emulate his father's advice of giving the enemy the appearance of victory (short-term) until the Corleone's could re-group. It was similar, albeit on a smaller scale, to the victory Vito appeared to give Barzini and the other families at the sit down meeting to end the war.

Another factor was Michael simply trying to reign in Vincent, who made his intentions of wanting to kill Zasa well-known. Remember Vincent's appearance at Michael's house the night of the killing of Zasa's goons: "I've got problems with the Commission, young man, and you don't make them any easier."

Michael's quest for true legitimacy was also being jeopardized when Vincent whacked Zasa. Michael wanted everything to look proper and legitimate when trying to take over for Immobiliare. He pled his case to Archbishop Gilday: "We’ve sold the casinos, all businesses having to do with gambling. We have no interests, or investments, in anything illegitimate."

It would not take much to see the connection of a murder charge (against Vincent) by one of Michael Corleone's relatives and how it could negatively impact Michael's ability to take over Immobiliare. "It was not what I wanted" was just a more eloquent way of saying "don't you dare screw this up for me" because of your conflicts with Joey Zasa.