An implication of monstrous bad behaviour.
Evil is a term used to de-humanise individuals. Sometimes written with capitalization.
Evil is the opposite of Good. A Bad Thing is not Good, but can generally be tolerated. An Evil Thing, on the other hand, is something that goes directly and deliberately against the grain of what many would consider ethically correct.
For example, the Recording Industry Association of America (RIAA) and other copyright representatives are thought to be evil, particularly by those who think it is better to freely share information and ideas.
Evil is a frequently-used term by the religious, because most religion-based ethical systems are extremely restrictive. Some consider Social engineering-driven advertisement to be Evil.