You should always approach your antagonists (or villains, or whatever you want to call them - basically the forces in opposition to your protagonist, or hero) in the same way you do your heroes: you should understand their motives and what they want should make sense. They way they go about trying to get what they want ought to make sense as well.
A film is so much more powerful and compelling if what the antagonist wants is in direct opposition to the what the protagonist wants. In other words: there's no way they can both get what they want. Only one can succeed.
But too many films just have a villain opposing the hero for the sole reason of creating conflict in the story, and if you really dissect the film and try to figure out the logic of the villain's plan, there is none.
I'm not a big fan of movies where the antagonist is just "evil" and wants to "control the world". As much as there are people in the world who crave unlimited power and money, they always have a specific reason why they turned out that way, and they always have a very specific overall goal they're heading for. But usually in movies where the villain "wants to take over the world" there's no thought put into making them like their real-life counterparts. I find it much more interesting and compelling when the villain is a rational person who feels they have every right to get what they are trying to get, no matter how outlandish their desires are. Because it's more true to the way people are: nobody walks around thinking "I'm evil, and I love being evil, how am I going to mess up the world today?" People are great at justifying and rationalizing their own actions so that they're the hero of their own lives and everything that they do is reasonable and even honorable from their point of view. People are really good at talking themselves into thinking they "deserve" to have pretty much whatever they want. People are even good at convincing themselves that when they do selfish things, they're actually doing them to help other people. People are endlessly fascinating and have an amazing capacity to talk themselves into believing things they really want to believe.
SLIGHT SPOILER ALERT from this point on...
Mother Gothel in "Tangled" is a good example to illustrate this point. At the beginning of the movie, she found a magical flower in the middle of a forest that kept her young. When it was stolen from her, she felt justified in stealing it back from the people that had stolen it from her. Sounds perfectly reasonable, right? .
She was a tricky character to work on, when it came to motivation. There were versions of the story when her "ownership" of the flower was much more legitimate. In early versions of the movie that we storyboarded, she actually owned the flower and kept it in her garden, behind a wall of her house (which she had built around the flower when she first found it hundreds of years ago). In that version, everyone knew about the magical flower. The King, when his pregnant wife fell ill, came begging for help in curing his sick wife. He offered Gothel anything for the flower, or even a piece of the flower, but being selfish, she refused him.
The King walked away empty handed and heartbroken. Unbeknownst to the King, later one of his men broke into Gothel's garden and stole the flower. The King used it to heal his sick wife, and the baby is born.
In retrospect, this may seem like a silly thing to have tried out. You don't like the King much for using a flower that was stolen, even if it wasn't done explicitly on his orders. But we thought it might work and still enable the audience to see Gothel as the villain of the story, because her reaction to the theft of her flower is to kidnap the King and Queen's baby. Kidnapping a baby is so much worse of a crime than stealing a flower and we thought that would make her seem much more evil than the King (or his men) in the audience's eyes. Also, the King's motives for his theft seemed more altruistic: Gothel was using the flower to stay young and alive in an unnatural way, whereas the King wanted the flower to save an unborn innocent. So what the King was doing seemed ultimately more noble and honorable than what Gothel was up to. So we thought it might work, and the audience will still like the King and root against Gothel.
But when we screened that version of the movie in storyboard form for the studio, a lot of people had sympathy for Gothel and the fact that she was the victim of a theft, especially because the first crime - the one that started the whole thing - wasn't committed by her, it was committed against her. Many viewers sent us notes saying that they had so much sympathy for Gothel and they felt so bad for her having her property stolen that they never had any empathy for the King or Rapunzel. To them, Gothel was in the right and was, in some ways, the hero of the story, which was throwing everything off.
Although this wasn't what we intended, I wasn't totally discouraged by this happening to the audience; I liked that people had some understanding of Gothel's motives and some measure of sympathy for her, because it meant that we were creating a very real antagonist, with real humanity and with real reason for doing the awful things she was doing, and not just an evil witch who was doing horrible things for evil's sake. But at the same time the audience has to feel more for the King and Rapunzel than they do for Gothel, so we were going too far. We had to tip the scales somehow.
So we carefully adjusted things to make the King less complicit in a crime against Gothel. We took the flower out of her garden and put it out in the middle of the forest, where it can't be considered anyone's "property". Even though Gothel took pains to keep it hidden, when the villagers who are looking for it come upon it, we orchestrated events so that there's no sign that anyone has been using it. And we didn't have Gothel confront them or try to stop them from stealing it. So they couldn't have known that anyone else had ever discovered the flower. All that helped to make it feel less like a theft of private property, and just a case of people finding an herb growing wild in the forest and taking it home to use for medicinal purposes.
Also having all the guards and villagers go to look for it helped put over the point we were trying to make - instead of the King and his guards going to get it, if the whole kingdom goes to look for it, it feels like the entire population of the Kingdom loves their King and Queen and by extension the audience likes them and has empathy for them. They must be good rulers and kind to their people, after all, if their people are willing to go to all that trouble for them. So that helps in making the viewer root for them.
But there were always some people that felt that we were making Gothel too human and understandable in her motives. While the film was being made (and even after it was released) there were people that told me that they felt we should have made Gothel more of an "evil witch". They felt that Gothel should have not been nice to Rapunzel and not bothered to masquerade as the girl's mother. After all, they would say, shouldn't she just chain Rapunzel to the wall, throw her bread and water once in a while, and use the hair whenever she wants?
There are a few reasons why I don't think that was the way to go, and why we went the way we did.
First and foremost, that approach is very dark and unappealing. It basically becomes a "Saw" movie. We were already telling a story about a kidnapping and a girl locked in a prison for eighteen years. No matter how you handle it, that's a dark and terrifying concept, especially in a movie that we wanted to appeal to all ages. So we thought it would be more palatable to the audience to make the prison a "gilded cage" - a place where the surroundings are really nice and the girl has everything she could ever need. It becomes a nicer place to look at visually on screen, and makes Gothel smarter - she's given the girl everything she could ever want, hoping to keep the girl stuck there and never wanting to leave.
(Also it's interesting to note that it's true to the concept in the original fairy tale, where the evil fairy gave Rapunzel everything she ever wanted, including a magical wardrobe that created whatever dresses where in style at the moment).
We also considered that Gothel might relate to the girl in the same way that she related to the flower originally. In order to encourage flowers to grow, people make sure they have all the water, sunlight and nutrients they need. People even talk to and play music for flowers to help them grow.
If we had created a situation where Rapunzel is kept like a prisoner in a dark and horrible tower, then it's an entirely different movie. She would already be set in her mind to escape from the beginning - there's no internal struggle between whether she should go or not, which I think is emotional and entertaining. And in that type of scenario, when she gets her chance to escape her imprisonment, she will run and never look back. There's not a lot of emotion to that type of movie - it's more like a prison escape story - and that's not the movie we wanted to make. Also our approach made our characters a lot smarter, in my mind: Gothel knows any normal person will have questions about the outside world and want to see it at some point. So why not create a wonderful home that's comfortable and will be that much harder to leave? And when the real world isn't as nice, won't Rapunzel be more likely to want to return? Also, we wanted Gothel to always be undercutting the girl's confidence in subtle ways, and filling her with fear about the world, so that she's doing all she can to keep the girl in the tower while appearing to be a nice, caring mother who's concerned for her daughter's well-being.
We wanted Rapunzel to be a very smart person, first and foremost. If her home had seemed like as prison in any way we figured she would have run away at the first opportunity, or else she would have seemed pretty dumb. The same thing if her mother had been mean to her constantly...a smart person would run from that and never look back. So we thought it would be better, more interesting (and more appealing to watch) if Gothel pretends to love the girl, but in reality she doesn't care at all about her and everything Gothel is doing is meant to keep the girl right where she is.
Anyway, the point being that I much prefer to have villains (as well as heroes) that are grounded in reality and remind of us the people we meet and read about in the news every day: people with real motivation for what they do and not some cartoony unrealistic plot to take over the world. If you can make your antagonists seem like real people who have been pushed a bit too far by their sense that they've been wronged, or convinced themselves that they deserve something, and are going just a bit over the line in their pursuit of what they feel they are owed, you can create a great memorable villain that feels real, and grounded, and all the more scary because they remind us of real people that actually do that kind of thing in our world every day.