Michael: You seem to be assuming certain characteristics of the soul in forming your question, and I believe these assumptions are limiting your understanding of the Catholic position.
Every human soul has its own absolute, unique will. All souls are created equal by God, at least when it comes to the exercise of will. On every human soul, God imparts the Natural Law, an innate understanding of what is Holy and what is evil. This imprint exists regardless of our upbringing and environment, and can’t be removed from the soul. This is a gift from God, a Grace, that ensures that the human soul doesn’t exist in a vacuum at any point in its existance. The human soul always, always, has the ability to resist evil, regardless of the outside pressures; it is solely up to the individual.
Now we come to your assumption. It seems that you are assuming that this will is not absolute and unique, when our faith tells us otherwise. Why does one soul freely choose evil, and another freely choose good? They choose because they can, plain and simple. In Genesis it says that God created Man in His own image, but what is this “image”? God exists without a creator, without an outside influence. God freely chose to create the universe by His own Divine Will, by Divine Fiat. Nothing influenced God in doing this, and nothing could have. God exists regardless of outside influence by definition. God’s name is “I am that is” for a reason; when God told this to Moses, He was exposing the core of His nature. In making us, God shaped that aspect of Himself into our being. When each soul is made, a little bit of “I Am” is right there in it. So yes, there IS a source, and an outside influence for each human soul, but that influence is the imprint of the independent Will, independent even from God. That is God’s image that we are made in.
So, we choose because we can. We choose by fiat, an arbitrary decision, and we can make such decisions by virtue of the fact that God made us with that ability expressly imprinted on our souls. There doesn’t have to be a greater “why” for the decision that a soul makes, just as there doesn’t have to be a “why” for God creating the Universe, or existing for that matter. Now there are of course influences, callings, and impulses, but ultimately every moral decision is made by fiat, otherwise it is not a moral decision at all; every soul can choose the good regardless of outside influences, though we can certainly lie to ourselves with the aid of evil influences.
Is it distressing that some souls freely choose evil? Absolutely, and that’s what makes evil so abhorrent! Is it frightening that some souls freely choose Hell (and the Church teaches that Hell is absolutely a free choice)? Certainly, and that is why all good-thinking people are called upon to call others to good. The choice is ultimately theirs, but we need not suffer the loss of our brethren to evil laying down. God has also made us capable of calling to one another, for both evil and good ends. Some philosophers seek to soothe the angst that evil causes in others by putting an ultimate outside “cause” for such choices, but Catholics can enjoy no such illusions. A soul is damned not by determinism, nor by random chance, but by absolute exercise of the will. Yes, it’s scary. Evil is scary.