Evil is the violation of, or intent to violate, some moral code
Morality is the differentiation among intentions, decisions, and actions between those that are good and bad . A moral code is a system of morality and a moral is any one practice or teaching within a moral code...

. Evil is usually seen as the dualistic opposite of good. Definitions of evil vary along with analysis of its root motive causes, however general actions commonly considered evil include: conscious and deliberate wrongdoing, discrimination designed to harm others, humiliation of people designed to diminish their psychological needs and dignity, destructiveness, and acts of unnecessary and/or indiscriminate violence that are not legitimate acts of self-defense but aggressive and designed to cause ill-being to others.
Unanswered Questions

Now you see, Lone Starr, that evil will always triumph, because good is dumb.

Dark Helmet, Spaceballs

One, who enters the places of evil repute has no right to complain against a man who speaks ill of him.

Ali, a hundred sayings

Jamais on ne fait le mal si pleinement et si gaiement que quand on le fait par conscience.

Men never do evil so completely and cheerfully as when they do it conscientiously.

He who studies evil is studied by evil.

Ranjen Solbor, "The Changing Face of Evil", Star Trek: Deep Space Nine, inspired by the famous quote of Friedrich Nietzsche

There is Good and there is Evil and Evil must be punished. Even in the face of Armageddon, I shall not compromise this.

Rorschach Watchmen

Without followers, evil cannot spread.

Spock, "And The Children Shall Lead", Star Trek

Good can imagine Evil, but Evil cannot imagine Good.

W. H. Auden, A Certain World

A thing may look specious in theory, and yet be ruinous in practice; a thing may look evil in theory, and yet be in practice excellent.

Edmund Burke, Impeachment of Warren Hastings, 19th Feb. 1788.

Some men aren't looking for anything logical. He can't be bought, bullied, reasoned, or negotiated with. Some men just want to watch the world burn.

Alfred, The Dark Knight Quotes reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895).