6 ideas
The harm a perpetrator does is almost always far smaller in the perpetrator's experience than the loss is in the victim's. Perpetrators gain little and move on quickly. Victims lose a great deal and remember it for years. This gap means both sides honestly misjudge what happened, and it makes cycles of retaliation grow because each side sees the other's response as out of proportion.
People picture evil as a force that is outsiders, sadistic, cruel for its own sake, and aimed at innocent victims. Real perpetrators almost never fit this picture. The myth persists because it lets victims and observers avoid moral complexity and deny any resemblance to wrongdoers. Seeing the myth as a comforting story makes it clear that most evil comes from ordinary people who think their actions are justified.
Violence and cruelty come from four sources: instrumental gain (harm used as a means to money or power), threatened egotism (a response to damaged self-esteem), idealism (harm justified by a higher good), and sadism (pleasure in hurting others). They are listed roughly from most to least common. Sadism, the cause people imagine first, accounts for only a small share of real evil.
Aggression is not mainly caused by low self-esteem. It comes from favorable self-views that are unstable or unearned and then get challenged by others. When an inflated self-image meets an insult or criticism, the person lashes out to reject the threatening evaluation. This explains violence by bullies, tyrants, and groups who believe they are superior.
People who believe they are serving a morally supreme goal, such as utopia, God's will, or national purity, can justify almost any means. The higher the goal, the more acceptable the cost seems. Idealistic evil also tends to be carried out by groups, which spreads responsibility and silences individual doubts. This is why the largest atrocities in history were mostly committed by people who thought they were doing good.
Most people already feel impulses toward aggression, so the key question is why restraint fails rather than why violence starts. Evil becomes likely when inner brakes weaken through exhaustion, alcohol, escalating small steps, group settings, or rationalizations that cancel guilt. Cruelty can also get easier with repetition, and a minority of perpetrators may come to enjoy it, which is how sadism develops.