Villains often do malicious, devious acts because of the negative influences in their own lives. However, most villains do not believe they bad. They believe they deserve to win because of what have been done to them in the past.
Don Corleone, in The Godfather, a sympathetic villain, was driven, at least in part, by his love for his family and his love for The Family (the Mafia).
Hannibal Lecter in Silence of the Lambs was driven by intelligence, boredom, and the knowledge he could outsmart his captors so easily.
Villains suffer from a narcissism, an inability to see and respect other people's reality, an inability to recognize the humanity of other people. Villains are often blind to their effect on other people.
A villain is not exclusively a contrast to the protagonist, to do the opposite of what they do, or to stand opposed to them no matter what. In fact, it is often convincing to have a villain who does evil because they want to cause chaos. As the author, you don’t need to agree with the villain's point of view but you should understand what makes villains evil.
If a villain is going to be defeated, readers want it to be on more than just a physical level. They want the villain to admit his defeat. Knowing how far your villain is willing to go to win is important to establish early on so you can create readers’ expectations. What your villain wants is almost as important as what your protagonist wants.
Understanding your villain’s most important desire and their deepest fear will give you both the character's motivation and their major weakness. Your villain should be well matched with your protagonist. In fact, readers should begin to think the villain could actually defeat the protagonist.
A weak villain robs the story of suspense, but one who is entirely too strong will be unbelievable. Villains justify their actions and paint themselves as simply misunderstood. While it doesn’t need to make sense to you as the writer, it does need to make sense to the villain. A particularly interesting villain can be someone who has turned to the dark side due to disillusionment or temptation.
Do other characters in your novel know this person is evil and working against their best interests, or are they being deceived? You should not share all this information with your readers, but you should know the villain as you write the book.