7. Pursuit of the good. The road to hell is indeed paved with good intentions. One should never underestimate the amount of evil caused by people thinking they were doing good. Far more evil has been perpetrated by idealistic people than by cynical criminals.
8. Sadism. There are people who simply enjoy seeing others in pain and inflicting it on them. But sadism accounts for few, if any, large-scale evils. It accounts for many individual acts of cruelty.
9. Boredom. Boredom is widely underrated as a source of evil. Yet, it most certainly is. Lack of purpose, not a lack of things to do, is the source of nearly all boredom. People need meaning in their lives. And if they don't, they will pursue visceral excitement instead of meaning or seek meaning in evil causes.
I believe there is a tenth explanation that is greater than all the others and is particularly widespread today.
10. Victimhood. A lifelong study of good and evil has led to me conclude that the greatest single cause of evil is people perceiving of themselves or their group as victims. Nazism arose from Germans' sense of victimhood -- as a result of the Versailles Treaty, of the "stab in the back" that led to Germany's loss in World War I and of a world Jewish conspiracy. Communism was predicated on workers regarding themselves as victims of the bourgeoisie. Much of Islamic evil today emanates from a belief that the Muslim world has been victimized by Christians and Jews. Many prisoners, including those imprisoned for horrible crimes, regard themselves as victims of society or of their upbringing. The list of those attributing their evil acts to their being victims is as long as the list of evildoers.
This is also true in the micro realm. Family members whose primary identity is that of victim usually feel entirely free to hurt others in the family. That is why psychotherapists who regularly reinforce the victim status of their patients do the patient and society great harm.
If my belief is even partially correct, the preoccupation of much of America with telling whole groups that they are victims -- of racism, sexism, homophobia, xenophobia and classism, among other American sins -- can only increase cruelty and evil in America.