Shorter Megan McArdle: Why does evil exist? Is it possible that good people could do bad things?
That's our Megan. Ignore thousands of years of debate in philosophy and religion to idly wonder how something could possibly be so.
UPDATE: A commenter inadvertently provides an answer to Megan's question, how could white people have been so cruel to black people in the old, old days? He says the bus driver could have been the reason, who would have been too scared of the law to fight discrimination. This is the authoritarian mind at work. It seeks to excuse away callous behavior and assumes everyone bows cravenly to authority. People do this because they were raised to obey authority unquestioningly, and consequently cut off their own emotions to deal with the difficulty of repressing themselves in the service of others' needs and wants.