Evil is seductive, and to drive that point home, bad guys can be elegant, creepy, or sexy. Some bad guys even combine those traits in different combinations, giving you creepy-sexy bad guys as just one example.
To further underline the seductiveness and underscore the temptation of evil, some Evil characters also seem to share a certain kinetic trait: body language that implies a certain gravity-defying elegant grace, a boneless, flowing, fluid manner of movement.
It can be sexily alluring, or unnervingly disturbing, but serves as a visual cue to a viewer that a character who moves in this manner is sliding away or completely removed from what we consider "their humanity", and as such is not a sympathetic character to root for.
In more supernatural cases, the villain does literally and voluntarily shift from solid to liquid or gas at will. Villains who are animals can display this trait honestly as it will often be a species trait: eels, reptiles (especially snakes), weasels, cats and rats all slink.
Occasionally, due to them not being able to help how they move — and bad reputation — you may encounter slinky animals who are neutral or good despite what it may look like when they move.
The trope is most commonly applied in animated fare, but it does make appearances in live action as well.
- Aladdin: Jafar worked the slink, and in the climax of the first film, took it all the way to transforming into an actual serpent.
- The Great Mouse Detective: Ratigan is an actual rat.
- Hercules: Hades glides effortlessly on Fog Feet.
- The Little Mermaid: Ursula the sea witch slunk like a pro on her octopus-like tentacles, even using her cave walls to really push the creepy and menacing effect. Flotsam & Jetsam are eels, coming by the slink naturally.
- Monsters, Inc. (excluding Monsters University). Randall Boggs was a decent guy for most of the prequel, and hadn't really developed the slinky way of moving until the events of the first film.
- The Princess and the Frog: Dr. Facilier and his "Friends on the Other Side". Justified in their case, as they were literally nonhuman and granted some of that to their charge, until he lost their good will.
- Sleeping Beauty: Maleficent had a more regal, gliding way about her, but still slinky.
- The Lion King: Scar is a big cat, thus this is somewhat inevitable, but his motions are much more slinky compared to the good lion characters who are brawnier and walk more elegantly.
- The Jungle Book: The the sneaky, hypnotic snake Kaa and the vicious tiger Shere Khan as the two main antagonists. The first one appears in two scenes where the animators clearly had a lot of fun animating his slithering motions, and the other is introduced in a scene where he crawls through tall grass to stalk a deer.
- The Book of Life: Both death gods move slinkily, but cheating, sneaky Xibalba more than kind-natured La Muerte.
- Cloudy with a Chance of Meatballs 2: Chester is briefly seen spying on Flint and his friends, slinking on a branch like a python.
- FernGully: The Last Rainforest: Hexxus was literally non-solid. He was oily sludge, and smoke, and moved like it.
- Sinbad: Legend of the Seven Seas: Eris used the slink to disturbing effect, randomly growing her face and body out of her constantly writhing mass of hair.
- Rise of the Guardians: shadowy bad guy Pitch Black.
- Terminator 2: Judgment Day: The T-1000 was made of liquid metal, and flowed from one form to another until it was too badly damaged to do so smoothly anymore.