Evil is the opposite of good. We usually think of villains as evil — wrong, immoral and nasty on many levels — and heroes as good.
This word is often reserved for our most terrible crimes and criminals: Adolf Hitler is usually the face of evil in history, while Voldemort is that face in recent literature. Evil can be less extreme, meaning simply anything that brings harm; we can talk about the evils of alcohol or the evils of power. When both major candidates for President seem lame, people often say they're choosing between the lesser of two evils. Not all evils are equally evil.