In most genres of story, the protagonist has an internal shift from the beginning of the story to the end of the story. They believe something different, they have new skills or ideals, a new point of view. There are a few genres, like sword and sorcery or super hero stories, where that can happen but doesn’t always happen. Conan may learn something exists, like in The Tower of the Elephant, which does make him a little wiser about his place in the universe but the central spirit of Conan never changes. In super heroes, you typically only get that sort of major shift in the hero’s origin story. Iron Man goes from being a selfish asshole to a less selfish asshole. Villains? Villains never change.
Villains have a purpose which they have set their minds to and are dedicated to. No matter what, they will not compromise. In Big Trouble In Little China Lo Pan wants to break the curse that has been placed on him. In order to break that curse he will murder, steal, abduct women to force them into marriage, use black magic and human sacrifice.
This is one element of what makes a villain “evil.” They want something that will only benefit themselves. What they want is going to cause others to suffer and they don’t care. Basic villains are selfish, lack empathy, and do not care that their desires are going to cause others suffering. Monsters are the most obvious and simple version of the concept.
The Terminator will never stop in its mission to kill Sarah Connor. The Alien wants only to eat and breed. Zombies always attack the living. The kaiju want to smash cities. These sorts of antagonists are fairly easy to create. You give them a desire or intention that is an existential threat to the protagonists, figure out how the monsters are going to achieve their intention and you have a conflict with high stakes.
Building an interesting and engaging villain that is a thinking person requires a little more effort. The key elements are that the villain has to have identified a real problem, their solution to the problem is “evil” and there is no compromise. Those antagonists who are sentient, who are not compelled by their nature and capable of reason will not be swayed. They have their point of view and have identified a real problem. The conflict is that their solution is going to cause a problem for the protagonist or the people they are trying to protect.
Consider Thanos from Marvel’s Avengers movies. He identified the problem of environmental destruction and the only solution he can see is exterminating half of the sentient beings in the universe. Thanos is absolutely certain that in order to save the universe, that he must kill half the population of sentient beings. His purpose is so important that he will sacrifice his most powerful minions and his adopted daughter to achieve the goal. Even his catch phrase, “I am inevitable,” tells us how clear this mission is in his mind.
Whether you are creating a villain for a game or a story, the fundamentals here are the same. Your villain has to have a goal; a purpose that may have good intentions but will have a cost which the heroes will not accept and fight to prevent the villain from achieving. Once you have that, you can then decide the resources and the lengths the villain will go to in order to achieve their goals. This produces the conflict and the stakes you need to create an engaging narrative.