Both were honorable men early in their lives but not extraordinary. Vito chose a life of crime when Clemenza took him to get a new rug.
Same with Michael. He chose the family business when he volunteered to avenge his father’s shooting, and turn his back on the path his father had dreamed for him.
That was when the opportunity to begin a life of crime presented itself - but it was not the "motivation."
Prior to that, he had himself been victimized by the "Black Hand."
Regards,