In the wake of grave evils, the most recent of which being the bombing at the Boston Marathon, people usually want to know, "What causes someone to do something like this?" How could two young men—one a husband and father, the other barely out of high school—plan and execute a crime that has caused three deaths and injured nearly 300 more?
Many look around for external triggers. In the days immediately following the tragedy, there was speculation that the bombers might have been motivated by Christian extremism or radical political conservatism. Now that the Tsarnaev brothers have been identified as Muslim immigrants from Chechnya, their actions can be explained away as jihadist terror and we can distract ourselves with brooding over the dangers of radicalized Islam.
It is so much easier to find an external trigger than to ponder the mystery of evil that can corrode each and every human heart, not limiting itself to extremists, either of a political or a religious bent.
All persons who have the use of free will have the power of choice. Angelic persons are pure spirit, and therefore are not subject to change. That means that the choice they made for good or for evil was an irrevocable choice, and where there is no possibility for change then there is no possibility for repentance:
Scripture speaks of a sin of these angels. This "fall" consists in the free choice of these created spirits, who radically and irrevocably rejected God and his reign. We find a reflection of that rebellion in the tempter's words to our first parents: "You will be like God." The devil "has sinned from the beginning"; he is "a liar and the father of lies."
It is the irrevocable character of their choice, and not a defect in the infinite divine mercy, that makes the angels' sin unforgivable. "There is no repentance for the angels after their fall, just as there is no repentance for men after death" (CCC 392–393).
Human persons, because they are embodied spiritual souls (i.e., souls that are pure spirit within material bodies), are subject to change. Because they can change, then until death separates the soul from the body, a human person can repent and be forgiven. Once death occurs, there is no repentance possible after death. Someone once asked me in the Ask an Apologist forum at CAF why that is. Why is man bound forever at death by a fallible choice he made to disbelieve in God during life and might regret once he sees God after death? I said:
Why do you assume that a man who spent a lifetime denying God exists would suddenly want to be with God should he be confronted by God immediately after death? Why do you assume that the transgression of deliberately denying the existence of the infinite, eternal God is merely a finite, temporal transgression?
Something to keep in mind is the nature of choice. Angels, because they are pure spirit, made one eternal choice either for or against God. Because they do not have material bodies subject to the limitations of change, their choice was eternal and irrevocable [at the moment the choice was made]. Human beings, because they have bodies, can change over the course of their lifetimes; but radical, life-altering change from the shaping of a lifetime of choices is as rare as a deathbed conversion precisely because our choices shape us into who we are. At death, that lifetime of shaping through the choices we make becomes as permanent and irrevocable as the one eternal choice made by the angels.
All of our choices, large and small, for good and for evil, shape what we are willing to do and who we will ultimately be when we meet God. If I may use a material image to describe a spiritual process, it is almost as if our free will acts as fingers on wet clay, molding a pot that will either last for eternity or be eternally lost.
Perhaps a more helpful picture can be found in J.K. Rowling's Harry Potter series. (Note bene: Spoiler alert!)
At the very end of the last book of the series (The Deathly Hallows), Harry urges the series's archvillain Lord Voldemort to repent for the sins of his life, warning Voldemort that Harry has seen what he will be should he not do so. Voldemort cannot fathom what Harry is talking about; apparently he is quite literally unable to feel remorse or to repent for the evil he's committed and is shocked that Harry should suggest he be remorseful for what he has done. The choices Voldemort made over the course of his lifetime have shaped who he will be at the moment of death. As Dumbledore once explained to Harry, "It is our choices, Harry, that show what we truly are, far more than our abilities."
So it is with good and evil. If we have use of our free will, we are free to choose good or to choose evil. The small goods we do this day can mold us into the kind of people who will run toward disaster at the risk of our lives in order to help. And the small evils we succumb to this day—whether they be inspired in the name of religion, or spring from a desire to further a political ideology, or merely seem glamorous in the moment—can shape us into people who become capable of setting down backpacks loaded with bombs in the midst of innocent people enjoying a race on a lovely spring day.