Moral evil is 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, or any other evil event for which someone can be held responsible or culpable.
This concept can be contrasted with natural evil, in which a bad event occurs naturally, 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.