For every Luke Skywalker there is a Darth Vader, for every Sherlock Holmes a Professor Moriarty. Every protagonist needs an antagonist to play off because without evil, goodness means nothing. Suffice to say that everything your main character stands for, the villain will oppose and vice versa.
At this point it is important to note that not every antagonist is a villain. Often there will be several antagonists in a script who don’t have that evil edge required to be classified as a villain. Rather they are simply doing their job which results in them opposing the antagonist. For example your main character might desire a bank loan yet be refused because of a poor credit background. The bank manager is not doing anything evil, simply carrying out his duty as a good bank manager yet he is directly opposing the protagonist.
The villain will oppose the protagonist but in a more sinister fashion. While the protagonist may believe in freedom of speech the antagonist may be suppressing that right in people in an active manner. Indeed villains are often the most active characters in a good versus evil story. They’ll lie, cheat, steal, maim, murder, bribe and betray, anything to achieve their goal.
There is a tendency in Hollywood movies for the villains to have little to no motivation for their attitude and goals, serving only as a shadow to the good force of the main character. However the truly iconic villain will be every bit as motivated as the good guy. Remember that no-one in life considers themselves the bad guy, everyone has a rational and justified reason for their actions…even if only they understand that rationale.
Hitler’s name is often brought up as one of the most evil men in history. His motivation was to obtain the space and resources needed for the Aryan race to spread and rule the world. Thus humanity would be in a superior position, smarter and stronger, in his opinion. Many other people have shared similar beliefs about various races throughout history but it is the way that Hitler set about achieving his goal, killing millions of people in a tyrannical rampage that sets him apart as one of the most evil figures in history.
As a scriptwriter you need to explain the why the villains acts in the way they do. The two main reasons characters become villains tend to be through their own victimization at an early age, or a self-serving attitude leaving the villain with little to no empathy. The “victim villain” tends to be a re-actor, with a backstory to explain his evil traits. The “self-serving villain” tends to have a number of unconscious factors which have resulted in their maladjusted world view, just like Adolph Hitler as looked into above.
Recognise that no-one is 100% evil. Instead they will have a few positive traits littered amongst a black cloud of negativity. Remember Blofeld from James Bond movies, while he had an obsession with killing James Bond (in highly convoluted manners) and world domination, his Persian cat certainly wouldn’t have seen him as evil. He thought the world of his cat and treated it with love.
One thing that bonds all villains is a certain form of narcissism. A set of beliefs where they feel that there world view, their thoughts and their feelings are correct and more important than anyone else’s. As far as they are concerned they are the only human while everyone around them is a mindless robot taking human form, therefore they are “perfectly justified” in sacrificing such pawns to achieve their goal.
Everyone’s always wrong…except me!