Evil is a word that is overused. The "Axis of Evil" doesn't really qualify, not if North Korea can exit the club by turning over some reports. Villagers called witches "evil" when really they were just eccentric. Even not all serial killers are evil. Some have illnesses that cloud the picture.
To me, evil is something bigger and deeper -- harm directed toward others, through action or inaction, without remorse. Most people would call the Nazis evil. Yet evil can be difficult to label because there can be mitigating circumstances and exceptions. In some ways it is one of those things that you cannot define, only recognize.
"'Hate' and 'wars' are the most profoundly obscene four-letter words," a wise man once wrote. I think that "hate" and "wars" are apt icons of human evil.
My view is that societies dictate morality and see their own self-interest as comprising "the good" -- and those things and/or individuals who do not promote the well being of the acceptable social order are deemed "evil" -- and society attempts to purge them. In this way, a population may consider itself working toward the good, or "higher good" while their neighbors, competitors or enemies will hold a very different view. Societies are quick to deem competitors, or competing world views as evil rather than take the time and care to consider that there may be much to be learned by being attentive to an alternative point of view.
Evil exists worldwide. It is not a disease but is a matter of choice.
What is evil? Guantánamo Bay, Abu Ghraib, extraordinary rendition -- and our indifference to the plight of these "disappeareds."