Besides the movie scene,that is totally served by a sequel of murders while Michael states that he declines Satan and all his acts (tremendous filming),we have to consider Michael's character,which is best described in #2:
Michael was the kind of guy who never takes any risks.In order to be the leader,he doesn't hesitate to "sacrifice" or better not to forgive,even his on brother.When Tom asks him "You won Michael.Do you have to kill them all?",he replies "Not all.Just my enemies".His power is his reason.If any of the heads of the 5 Families survived,they would run after him.They might,but he doesn't want to take the chances.
Except only one time,for Michael it was not personal,not revenge,ont infamnia,not honour,it was just buisiness.

Quiz/pole: which was this very unique time?