Keep worries and complaining to a minimum. Avoid all forms of condemnation and other-worldliness. A child's mind cannot understand minute differences. It tends to think wholistically, thus any concept will be engulfed and not digested. The child will learn ethical philosophy by observing how you treat others around you.
Severity is sometimes necessary, mercy is sometimes necessary, and mildness is sometimes necessary.
Use your head. Think things out before overreacting to situations. Do not moralize with the child. Speak of actions and behaviors in terms of real consequences. Do not impart metaphysical concepts until the child is able to understand them and their uses.