Moral evil is the result of any morally negative event caused by the intentional action or inaction of an agent, such as a person. An example of a moral evil might be murder

Murder

Murder is the unlawful killing, with malice aforethought, of another human being, and generally this state of mind distinguishes murder from other forms of unlawful homicide...

, or any other evil event for which someone can be held responsible or culpable.

This concept can be contrasted with natural evil

Natural evil

Natural evil, or surd evil, is a term generally used in discussions of the problem of evil and theodicy that refers to states of affairs which, considered in themselves, are to be avoided and not to be promoted, and for which no agent is morally responsible...

 in which a bad event occurs without the intervention of an agent. The dividing line between natural and moral evil is not absolutely clear however, as some behaviour can be unintentional yet morally significant and some natural events (for example, global warming) can be caused by intentional actions.

The distinction of evil from 'bad' is complex. Evil is more than simply 'negative' or 'bad' (i.e. undesired or inhibiting good) as evil is on its own, and without reference to any other event, morally incorrect. The validity of 'moral evil' as a term, therefore, rests on the validity of morals in ethics

Ethics

Ethics, also known as moral philosophy, is a branch of philosophy that addresses questions about morality—that is, concepts such as good and evil, right and wrong, virtue and vice, justice and crime, etc.Major branches of ethics include:...

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Some other examples of moral evil: violence, adultery, dishonesty, foul language, slavery etc.