Jesus and the moral lesson of the parable of the good samaritan in the bible

The parable of the Good Samaritan is the most important parable of the gospels, it is an application of many teachings of the books of proverbs and psalms, is probable that Jesus, by giving this teaching he was inspired by these books: "The way fo the guilty is devious, but the conduct of the innocent is upright", proverbs 21:8; "Good and upright is the Lord; therefore he instructs sinners in his ways. He guides the humble in what is right and teaches them his way" Psalm 25:8-9
The most important moral lesson of the parable is that we can sin without violating any commandment, we have to choose between righteousness and sin, between the spirit and the flesh, the parable is not a case of simple mercy, that is a simple and erroneous analysis. Enter the kingdom of god is to accept the path of righteousness.
There is an opposition between the Levite and the Pharisee on the one hand and the good samaritan on the other. If the parable of the good samritan want to show us the importance of mercy, why the levite and the pharisee appears in the story? The teaching of the two paths is very clear.
The parable of the Good Samaritan summarizes the science of ethics: the good and evil are paths, ways to choose.
The gospels and the doctors of the church says so:

The parable of the good samaritan is in luke 10:25-37:
"A man was going down from Jerusalem to Jericho, when he was attacked by robbers. They stripped of his clothes, beat him and went away, leaving him half dead. A priest happened to be going down the same road, and when he saw the man, he passed by the other side. So too, a levite, when he came to the place and saw him, passed by the other side. But a samarithan, as he traveled, came where the man was; and when he saw him, he took pity on him. He went to him and bandage him wounds, pouring on oil and wine. Then he put the man on his own donkey, brought him to an inn and took care of him. The next day he took out two denarii and gave them into the innkeeper. "Look after him" he said, "and when I return, I will reimburse you for any extra expense you may have"
Which of these three do you think was a neighbor to the man who fell into the hands of the robbers?"