What you would want others to do unto you, do unto others. Murder is wrong. Good for their own sake. The function of man is an activity of soul which follows or implies a rational principle. We become virtuous by doing virtuous acts.
Moral virtues are means between extremes.
We must do the right thing in the right circumstance, in the right way, and for the right reason. A good will acts on principle. If . . . then you should . You should . . . A first principle must be categorical.