The word villain is inevitably associated with evil, but what is evil, exactly? Generally, the definition of evil in this context has to do with morality, or lack thereof. So, to understand what evil is; it helps to understand morality. And the best way to do that is to analyze the types of things people refer to as moral.
Simply put, morality/goodness is a selfish act/attitude which best serves the interests of a general community, or allows the majority of people to survive and flourish. Evil is a selfish act/attitude which tends to work according to the best interest of the individual, and possibly against the best interest of the majority. But wait, if something works in of the best interest of the community, how can it be selfish? The answer is that all human actions are inherently selfish; in that they originate from the desire an individual has to serve their own emotional, materialistic or other survival based needs. Don't think so? When you hear from a person who often does charity work, what do they say? "I enjoy helping people." I enjoy; you will never hear a sleazy businessman or apathetic couch potato, say "I can't stand dealing with this human pond scum, but I still give one day out of my week to do it, because it's right!" Slogans for goodness often use this line of thought, such as; be considerate to others, think of others just as you would yourself, and in many cases, various degrees of self sacrifice are considered noble.
So, it can safely be said that morality is a concept which is designed to supplement the greater good. Isn't that a good thing, you may ask; not just from a moral standpoint, but from a practical one? Well, that really depends on your priorities, is everyone's best interest your first concern or is your individual interest a priority? And more to the point are you willing to sacrifice your personal well being, and happiness for the sake of the many? Some may say that such a question is unfair, but it should be observed the reason it's "unfair" is because we are all selfish creatures which cannot hope to consistently live up to the moral guidelines which we, ourselves, have set. Another consideration is the simple fact that society is made up of individuals, and without them there is no "greater good". Therefore, it's not too unreasonable to demand certain levels of individualist advocacy, in fact, it's essential.
Many will argue that evil is more than simply being selfish, which sounds good as a general principle, but upon further analysis does not stack up to the practical realities of the world. These are usually the same people who will say that an impersonal force of nature can't be evil; it just is. Which raises the question, if a force of nature can't be evil, and a person working on behalf of their own desires/ self interest isn't evil, than what other possibilities are there? Isn't a serial killer who commits murders for his own enjoyment, simply being selfish in an extreme sort of way? He simply values his "recreation" over the lives of others. And what of a tyrannical dictator who commits genocide and unleashes the dogs of war on unsuspecting countries for his own personal power and agendas? Exactly, at what point does someone go from being obscenely selfish to completely evil? Could it be that the only measurement of this is how large of a destructive impact one has on society?
No one does evil for evil's sake, it's a ridiculous notion, and completely irrational; even by human psychological standards. So, unless you believe in a supernatural being which brings pure evil to the world (which is a ridiculous notion, though typical by human psychological standards), there is no other source for evil other than human selfishness, in one form or another. Untold volumes of books, plays, movies, etc. have featured characters that are forced to grapple with the classic "moral dilemma". The reason that this has been such a frequent topic of conflict is horrifically simple, because the same base motivations which cause us to do "good" cause us to do "evil". The part which most folks would prefer not to think about is the fact that one cannot even begin to draw a line between good and evil, it's not even blurred; it's the same freak'n space! That is a hard fact to swallow and most people never will.
One of the most basic, prerequisites to being a true villain is to understand and embrace this philosophical taboo; to know that there is no real good or evil, black and white, that it's all just gray area and most of it is the same shade. And, if you really think about it, what could be more hazardous to the status quo than that?