As with your protagonist your antagonist needs some serious motivators beyond, ‘I’m evil and I want to rule the world’. A convincing protagonist should have more motivation than ‘I’m good and I want to save the world’, for some reason I imagine them skipping through the field throwing petals from a basket.
Perhaps you’re thinking ‘What about all those stories where the villain really does only want to rule the world and/or universe.’ The first characters I think of are Loki and most recent Blofeld, both of whom are looking for world domination in different ways and both of whom have issues with their fathers. Loki’s issues seem to stem from a belief that as he is adopted his parents love him less than Thor and denied him what is rightfully his in favour of his less clever brother. Blofeld, on the other hand, seems to think that his father loved Bond more and therefore he hates both of them and appears to be trying to prove he is better. Arguably, we’ve got a modern villain cliché developing about father issues, Kylo Ren in Star Wars: Force Awakens is another villain with father issues. However, the point is that the characters are moving away from the earlier villains who were evil because they were evil. In earlier Bond films, for instance, he villains appear to have very little motivation beyond being evil and wanting to rule the world.
Another thing that needs to be considered is stereotyping. Villains are often born of what societies fear most and usually this ends up taking the form of racism and xenophobia. At present there are numerous Eastern-European gangsters or terrorists who are of Middle-Eastern descent and/or Muslim appearing in fiction, often with flimsy motivations that don’t go much beyond ‘I’m evil’. During the Cold War it was The Eastern Bloc, during The Second World War it was Germany and Japan, and in the Western it was invariably the native people. I could go on and on listing the number of countries/cultures/faiths that have been fiction’s primary antagonists but I think my point is apparent.
I’m not saying the villain can’t be from a different country/culture/faith from the hero but their characterisation must be as complex as the protagonist’s. They need to be more than evil for the sake of evil or on the basis of a xenophobic/racist stereotype who exists purely to be destroyed by the hero.
The same is true of mental illness, another popular villain trope. Although when I say ‘mental illness’ this rarely has any connection to a true depiction of mental illness but rather of a trope in the shape of ‘I’m evil because I’m “crazy”‘. Returning to Bond, the original novels are festooned with harmful tropes, the villain often wants to rule/destroy the world because they’re ‘power mad’ or ‘crazy’. Not only is this trope harmful but we can easily build a convincing motive for the villain without it, as we discussed above.
If we look at the trope as it is, rather than what actual mental illness actually is, what it comes down to is a villain who has no concept of right or wrong. We have to ask ourselves what is a more terrifying villain: One without a concept of right and wrong, or one that knows the difference and chooses to do the wrong thing? Or even one that chooses to do the wrong thing for the ‘greater good’?
While Stars Wars: The Force Awakens once again uses problematic tropes one thing is very clear about Kylo Ren. He knows the difference between right and wrong and actively chooses the ‘dark side’ at ever oppotunity. Loki is also presented with various moments where he can do ‘the right thing’ and every time he chooses to do the wrong thing and he knows it is the wrong thing. In The Avengers Assemble Thor gives him multiple chances to stop what is happening and every time he chooses not to because Loki’s primary motivator is love of Loki.
When writing the antagonist you need to ask the same questions you would ask about your protagonist: Who are they? What do they want? Why do they want it? How far will they go to get it?