I also think that Michael was much more sheltered then his other siblings. In the garden seen, a little before Vito dies, he tells Michael that he alsways wanted him to become a successful legitament man. So his father was probably trying to keep him out of the business completely. However, Michael did see some things, from the outside and from the outside the don life was all about respect and Michael was brought up thinking that that was what the mafia was about.
However, Sonny who lived and breathed the mafia, making his bones at 16 I believe, knew more about the mafia. I do believe that Sonny would help a guy out before Michael does. Its sad though Vito tried so hard to bring him up legitimently but Michale still ends up spiraling down from the one thing Vto didn't want him to be. This is kind of like how Michael, as hard as he tries to protect his family, the more he is slowly ruining it. See the pattern? So I guess we can all blame it on Fanucci for forcing Vito to kill.