Zaf-the-Don hits the nail on the head with his comment on hypocrisy.

TB is correct that Vito is portrayed as someone with a sense of proportion and justice. However, there are many indications that this presentation is erroneous. For instance, Vito was willing to kill a man for holding Johnny to the terms of a legal contract. Is that justice?

Surely, this is just one of many examples through the years of Vito's willingness to use violence to achieve his ends, regardless of proportionality.

Yes, Michael is more willing to kill. But that doesn't mean Vito isn't a liar and a hypocrite when he says "We're not murderers."