All people are capable of great good and great evil

Human beings have been both incredibly evil and incredibly good throughout history. People showed remarkable, selfless heroism in that theater in Colorado that far surpassed one man's inner darkness. The same is true for every tragedy, from 9/11/2001, back through history, from those who sheltered Jews and other people in the Nazi Holocaust to those who helped black slaves escape from the Ante-Bellum South, etc. etc. For every act of evil, there have been many, many acts of goodness. 

Of course, there’s always great evil among men and women, but we ALWAYS have a choice to act well or badly. Even Cain, the son of Adam, the Biblical First Man, was said to have been given a choice by God, and he was told he could act EITHER way: "If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but YOU MUST RULE OVER IT." (Gen. 4:7 my emphasis.)

Through this story, we learn that we always have a choice, and that we CAN and (if we believe in the God of the Bible) we MUST choose to do good. 

Jesus taught that we were made to obey God, and gave every indication that we could do so. In fact, our obedience, and our seeking of forgiveness from God when we fail to live up to His high standards, is a requirement of our eternal salvation.

For as John wrote, "By this we may know that we are in him: whoever says he abides in him ought to walk the same way in which he walked." (1 John 2:5-6.) That, along with Jesus's own admonitions and urgings to be perfect, to forgive as God forgives, and to seek God's kingdom here on earth, ought to solidify our belief that we enjoy God's precious gift of freedom of choice, and that we must use it to seek goodness in all our acts.