There are several scenes which demonstrate what kind of man the Don was....

1) The scenes with the landlord. When the landlord refuses to let the lady back in, Corleone suggests that he ask around the neighborhood to find our "how I return a favor," juxtaposed by the sheer joy he gets out of extorting him later on once he has checked the Don out.

2) After he kills Fanucci and someone gives him a free piece of fruit and he thanks the vendor and says "anthing I can do for you let me know.

3) The great scene when Genco's father offers him groceries and he turns them down.

4) The killing of Don Ciccion as someone above has already eloquently posted.