Hate to say it but, religion most likely. For some, it manifests a cult that can justify genocide, endless war, murder, oppression, etc. All it takes is one massively deluded being with the power and charisma to lead a flock of sheep to their own slaughter, taking as many others as they can trying to "cleanse" the Earth and "do god's work".
But I think there is a certain level of defect in the human race. People who can take anything that they see around them, and turn it into a reason or motive to destroy life. I don't think you can reform these types. I think we simply have to stop breeding them. Eugenics, in theory, could be utilized to weed out the kinds of people born with the likelihood of developing such a personality disorder as to commit suicide in favor of a god, or commit genocide in favor of an ideal race, or lifestyle. One can dream.