The film doesn't show much violence. We see mostly the top level gangsters. This is what leads Vito to the deteriorated self image: "After all we're not murderers." But murderers they were from the very beginning.
Clemenza would have killed the cop when they stole the carpet.
They would have killed the truck drivers they robbed if they didn't give in.
The novel and the film portray Vito as a mild mannered man. And this is what makes the character Don Corleone so interesting: The killer and the tender, loving man.