When a Christian falls into great transgression--or when someone like Cho commits the atrocities that he did at VA Tech--even Christians seem mystified at the capacity of others for evil. "How could a person do that?" is a question I am often asked, although no one seems to expect an answer. It must be drugs, or a mental illness.
These are the only answers left, because the sinful nature is ruled out. "But aren't people naturally good?" No!
In Luther's lecture notes on Genesis (monumental volumes well worth studying), he says original sin is manifested at every stage of human development. Children need "a heap of rods" to bring them to order; with adolescence comes increased rebellion accompanied by "the passion of lust and desire." When a man marries, he loathes his own wife and begins to desire strange women. Men in government are marked by jealousy, personal ambition, arrogance, greed, anger, and ill-humor. With each passing year, Luther says, sins increase. (See AE 2:119)
I see this in my own life most of all. I'm ashamed of who I am. I want to be a Christian, but it seems like I'm getting worse, not better. There's a reason why the Kyrie must be included at every Divine Service. What else is there to say? "Lord, have mercy on us!"
It is not at all surprising that some Christians fall from grace into the most horrible sins. What is surprising is that the world is not even worse than it is. That the flowers still bloom, that rain still waters the earth, that God doesn't destroy us all for our ingratitude and impiety - that is surprising. That is grace. What a wonderful surprise!