EVIL: part II in my 'foundational principles' series (part I is here). I wanted to do deontology next, but I can't really explain the way out of the main line of criticism without touching on this:
"What then is the missing component in Socrates' definition of sin? It is: the will, defiance. Greek intellectuality was too fortunate, too naive, too aesthetic, too ironic, too-- too sinful-- to be able to get it into its head that someone would knowingly refrain from doing the good, or knowing what is right, knowingly doing what is wrong."
-Soren Kierkegaard, The Sickness Unto Death
"No! Let's rather say an even harder thing: that evil may not be as far beneath our surfaces as we like to say it is. --That, in fact, we fall towards it naturally, that is, not against our natures."
-Salman Rushdie, The Satanic Verses
There are three types of people in the world:
1. People who do at least some morally unacceptable things at least some of the time (lying would probably be a fantastic example)
2. People who are, for reasons of complicated and deep-seated physiological and psychological nature (like people with OCD, for example) who cannot reasonably have moral praise and blame attributed to them.
3. People who are, in serious analysis, unwilling to follow the rules of morality, who present serious dangers to the people around them, and who cannot be 'cured.' (your Stalins and serial killers).
It's the reality of the first of these that makes interaction amongst people as complicated as it is, but it's the reality of the third which has to define our moral and political commitments.