Cain did not fall in a day, rather there were many choices that he made that led to his downfall. I had the students read Moses 5, looking for at least 5 steps that Cain took that led to his fateful decision. At each opportunity, there was a chance to change his heart, to forsake sin and come unto the Savior. Unfortunately for Cain, (and especially Abel!) he did not take the multiple opportunities to change.
Sin becomes a habit.
President Spencer W. Kimball put it this way: Sin is intensely habit-forming and sometimes moves men to the tragic point of no return. Without repentance there can be no forgiveness, and without forgiveness all the blessings of eternity hang in jeopardy. As the transgressor moves deeper and deeper in his sin, and the error is entrenched more deeply and the will to change is weakened, it becomes increasingly nearer hopeless and he skids down and down until either he does not want to climb back up or he has lost the power to do so. (Miracle of Forgiveness, p. 117)
Another problem facing those steeped in wrong decision making is not only the deep entrenchment of sin, but the sinners inability to see their choices as being a problem. The more you walk in the mists of the world, the harder it is to see clearly. CS Lewis put it this way:
When a man is getting better, he understands more and more clearly the evil that is still left in him. When a man is getting worse, he understands his own badness less and less. A moderately bad man knows he is not very good: a thoroughly bad man thinks he is all right. This is common sense, really. You understand sleep when you are awake, not while you are sleeping. You can see mistakes in arithmetic when your mind is working properly: while you are making them you cannot see them. You can understand the nature of drunkenness when you are sober, not when you are drunk. Good people know about both good and evil: bad people do not know about either (C. S. Lewis, Mere Christianity, Book III, Ch. 4, Para. 10, p. 87).