What's brilliant about these lines is how well it sums up the meaning behind these core concepts that govern our lives. What is Good? What is Evil? What is Right? What is Wrong? The only reason why we, as humans, understand these terms is because society agreed as a whole the meaning. We all agreed that the sound "Good" is associated with being morally right or righteousness because we unanimously agreed to interpret it in this specific manner.
But surrounding these clear and self-evident concepts is a thick border of grey which brings about different interpretations and opinions.
A person has just been killed, but perhaps the person was the next "greatest" dictator, a patient suffering untold pain who must be relieved, or the carrier of a deadly disease who must be killed to sustain the rest of the population. True to this line, no one can truly know these concepts.