Ah, villains. Antagonists. Yes, there is a technical difference between the two (and even a third category of just "obstacles") but still, for any good story, you gotta have a good bad guy.
There's been a lot of trends over the course of, well... human history. It feels like right now the trend is for bad guys that are as or even more interesting than the hero. And, you know, that's pretty awesome. I really love it when I sit down to a movie or a kick back with a book and think "Wow, I actually kinda like the villain. I see their point." And really, that's one of the big keys, these days anyway, is making sure the villain has a point to what they're doing. After all, they're only the villain to your protagonist. No good villain wakes up and thinks to themself, "Wait... am I the bad guy?"
Your villain needs aspirations that are as noble to them as your hero's. The villain needs to be able to justify their actions to themselves, whether it's just being able to see a bigger picture or being the only one willing to do a dirty job.
Now, that doesn't mean you can't have straight up, evil for evil's sake villains. Those are good, but so much harder to pull off. Without good motivation, without a desire that the reader can understand (probably not agree with, but at least understand) the villain can come off as campy or cartoony. Like they belong to a 1960's comic.
Weirdly enough, though, one of my favorite villains of all time actually is this kind of villain. She's from a weird little cartoon called Wander over Yonder and, honestly, this little song I'm about to share got me to watch the entire series (note: she's in season two.) She's a new take on the old villain trope of evil-for-evil's sake. Is it because the cycle is coming around? Is it because the song is so catchy? I have no idea, but I freakin' love her.
Anyway, big point is, if you can't make your villain believable, at least make them interest.
Be Evil to Each Other.
Keep your eyes open for my debut novel, The Paladin.