First, man is not an angel, nor is he a devil. He is not intrinsically corrupt (as theologians began claiming four hundred years ago) nor is he intrinsically divine (as philosophers began saying fifty years ago).1
Rather, man has aspirations to good which he finds it impossible to realize completely by himself; at the same time, he has an inclination toward evil which solicits him away from these ideals. He is like a man who is down a well through his own stupidity.
He knows he ought not to be there, but he cannot get out by himself. Or, to change the picture, he is like a clock whose mainspring is broken. He needs to be fixed on the inside, but the repairs must be supplied from without.
He is mistaken if he is an optimist, who believes evolution will give him a mainspring, or a pessimist, who believes that nobody can fix him. He is a creature who can run well again, but only if some watchmaker will have the kindness to repair him.
Second, this conflict has all the appearances of being due to an abuse of human freedom. As the drunkard is what he is because of an act of choice, so human nature seems to have lost the original goodness with which a good God endowed it through an act of choice.
As St. Augustine said, “Whatever we are, we are not what we ought to be.” The origin of this conflict has been told by medieval and modern theologians through the analogy of music.
Picture an orchestra on a stage with a celebrated conductor directing the beautiful symphony he himself composed. Each member of the orchestra is free to follow the conductor and thus to produce harmony.
But each member is also free to disobey the conductor. Suppose one of the musicians deliberately plays a false note and then induces a violinist alongside of him to do the same. Having heard the discord, the conductor could do one of two things. He could either strike his baton and order the measure replayed, or he could ignore the discord.
It would make no difference which he did, for that discord has already gone out into space at a certain temperature at the rate of about 1100 feet per second. On and on it goes, affecting even the infinitesimally small radiations of the universe.
As a stone dropped in a pond causes a ripple which affects the most distant shore, so this discord affects even the stars. As long as time endures, somewhere in God’s universe there is a disharmony, introduced by the free will of man.
Fulton J. Sheen, Go to Heaven: A Spiritual Road Map to Eternity (pp. 37-39). Ignatius Press. Kindle Edition.