They lied to themselves all the time. Of course, they were murderers. They needed this whole honor-myth (as well as the "It's only business"-talk) to disguise and justify their crimes.
Yes and no. This is the dilemma which haunted Michael all of his life. Was he really doing murder, or was he protecting his family? Is it really murder to kill someone who is trying to kill you or your family?
I think that the murder of is brother is really the only one he felt was a mistake.
