There is one rule of writing that is very necessary, and yet very simple.
Your hero wants something. Put something in the way of their getting it.
This is typically titled conflict. Conflict is what drives your plot onward, and your character toward change. Conflict comes in many shapes, form or fashion, from the rain-clouds keeping the protagonist from going on a photo-shoot, to the villain trying to take over the world. Conflict is most often brought about by the antagonist who, in turn, is most often a villain.
People, especially younger authors, often tend to forget to develop their villain. Whether they forgot to ask the ever-needed question of “Why?” or they forgot to remember that their villain is a human being, capable both of failings and successes and, indeed, hopefully less likely to make unintelligent mistakes than the hero (I refer to the previously stated rule), villains tend to be left out of the list of important things to work on.
There are some people who make their villains evil beings with no motives, and then there are others who raise the cry of, “Villains are people, too! They were caused to be bad by their early lives! The poor things!”
Villains are people. They have their hopes and dreams and, yes, even fears. They might not fear your heroes; they might fear fire, or water, or being alone. Yet God gifted humans (and other sentient creatures!) with the incredible gift of free will. Animals respond to how they are raised and taught, but humans always have the choice to hate, or to love. Villains are those who chose the first.
So I ask you the questions: Why does your villain do what they do? When they were a child, what were their hopes for their lives? What do they fear? Who do they love, whether now, or in the past? What causes them pain? And, for a fun option, is there a song that makes you think of your villain? Comment!