When creating a villain one has to understand how an evil mind works. It is strange when writing a character who kills; how we become attached to them. We create the motivation that causes them to kill. Yet we have to foil his plan. It is a slippery slope for an an author. It's strange how we become fond of them. How twisted their mind. Becomes our mind as the reader. I am a huge fan of a good villain. Disney is a one company that creates great Villains. The evil queen, Frollo from hunchback of Notre Dame, Maleficent from Sleeping Beauty; to name a few. They do such a good job of creating a story that is plausible we understand the villains and their motivations. As a writer we must follow the same guidelines, and reasons for the bad guys to do the things they do. We must create fear for the life of our main character. Fear can be created through emotional conflict, family conflict, or physical conflict caused by villain.
In that fear the villain must be a very good at his evil deeds. Blackmail, threats, physical harm to increase the fear.
Otherwise our attention is lost. We lose interest in the strength of our hero or heroine. They must overcome this obstacle, this villain. Sometimes emotional conflict is more difficult then physical harm. Sometimes a character can be there own worst critic, villain. 
As I am creating a villain that will carry through in four novels. I'm finding different elements to carry him through all four stories. It is challenging especially in a series, to find a way for the good guys not to find him. But there must; be a good reason why. It must; make sense otherwise we lose the reader. If you the reader do not believe that this villain can truly succeed, or that our hero can overcome him there is no point to the story. Without our twisted villains, are antagonists; that create all of this conflict, suspense, our story is nothing. Even without a serial killer, or some action to cause our character to fail before they reach their happy ending there is no story. That my friends is the key to a great story, internal conflict, emotional conflict, physical conflict, these are all things that propel our hero forward. These are also things that propel our villain forward. If these are not in the story there is no story. So in conclusion, make sure your Villains are as developed, with backstory, motivations, conflicts, as your Heroes it will make the story more interesting, and you as readers will enjoy the story more. 
~Writing as in everything is a work in progress...Until next time readers! 😃 Jess~
