Making a good villan is one of writings challenges.

I always find that to create a character that you want people to percieve as the "bad guy" the majority of the work goes before you actually introduce your character.

Take Hannable Lector for example. Before you even meet him you're told stories of his behaviour, he's locked in an assylum, Starling has a claustrophobic elevator ride down to see him where she's shown a picture of a nurse he disfigured and how calm he was when he did it. She has to go through multiple gates and walk solitary down a long character of crazies before she see's him standing perfectly still and presentable in the middle of his cell. Yet he is polite and courteous. The whole scene of Lectors introduction is unsettling which reinforces the characterisation of him as the villan.

Because of the effort made in presenting this character, Lector made up less than 20 minutes of the movie, but stole the show. In the book Thomas Harris has more time to work on the character, but the end result was the same.

An evil character is one that the protagonist is fearful of yet must defeat. A strong characterisation will mean a lot to your story, but an archetypal villan has one very basic rule to follow. They must oppose that which your protagonist stands for.

I'd say think of the villans you've admired most in books and movies and see how they were represented. What was it that made them appear "evil" and keep that message reinforced in the character for the duration.

Villans are always more theatrical than heroes, so where a hero must prove their virtue to an audience, the villan can be built in imagery and emotion before they ever reveal themselves to the audience. Don't forget the old horror axiom, "its not what you show, its what you don't show".