Soulless Beings is a trope found in some fantasy, science-fiction and supernatural works of fiction, where the source of a villain or other character's evil and/or amorality is that they are intelligent entities who do not possess any soul.
Some Soulless Beings are naturally (born) soulless, while others originally had souls but have been separated from them.
Some elements in this page based from two TV tropes: The Soulless and Soulless Shell.
Lacking any/Loss of Soul (Became Soulless)
When people use the phrase "soulless monster," they usually mean the person they're referring to isn't acting like a (decent) human being, not that they actually lack a soul and are a monster* . In fiction, some characters, even villains, really are soulless, and often act like monsters because of it.
Soulless villains are motivated by one thing only: getting a soul, as it's mainly said they used to have one, but lost it for various reasons (consequence for made a deal with the Devil, forcefully extracted from their body, certain curses that reap their soul away without killing them, etc) but somehow remain lived. Any old soul will do, but frequently they want their own soul back for sentimental reasons. While a soulless character doesn't necessarily become a soulless killing machine, sanity and humanity don't fare very well without one. Even if both of these traits are independent of the soul and don't suffer in its absence, most Soulless characters have a change in worldview that does erode their former nature, if they have one.
Things can always get worse, of course. The loss of a soul enables villains to freely jump headfirst down the slip and slide of The Dark Side, or simply smile and carry on as if nothing happened... and kill anyone who disagrees. Like an appendix, it was just a useless organ weighing them down. If they lucky, its absence will be felt before long. Their allies eventually leaves them because they aren't treated the same, they don't feel happy or sad. What they do feel is a keen emptiness that gnaws on their conscious mind like an ever-growing black hole slowly syphoning a star's outer layers. With any luck, they'll try to get their soul back using mostly moral means.
In another case, however, the individual also revealed/said technically born/created without any soul. This drove them to obsessed to getting a soul so they can perfect their own essence, like Fabrication Machine.
As Soulless Shell
This case is similar with above, but the villain whom become one of soulless shell usually having their body restored/remain intact after their demise, and lacks any will to regain their lost soul. After revived/brought back to life, they may looks like their former selves, as well as breathes and moves like they used to be. But there's nothing inside. They just like a puppet. Their eyes are frighteningly empty. Whatever magic or technology used to bring them back couldn't quite finish the job and couldn't restore the essence of what they was to the body. They are no longer a villain whom both heroes and other villains known. Now, they are just a thing masquerading as their former selves.
Villains that became soulless shell, like The Soulless retain their goal despite their twisted nature albeit lack any will to retrieve their lost soul.
Rare case of soulless villains, these villains portrayed as entities that said to lived since ancient times or since the dawn of creations. Unlike cases above, they had no difficulties to lived without any souls at all. Some even shown being act or think like normal entities that has soul, which made them difficult to differentiate with those who has soul. These entities sometimes portrayed as complete monster or dangerous enemies to heroes, as some of this villain shown with powers to took other being's soul which also enable them to amplify their powers. Perfect example of this was demons in Buffy the Vampire Slayer.
Although, beings that can be considered naturally soulless not always have bodies, as some of these are already spiritual beings. This means that nature of their physical manifestation are not so different with nature of soul of living beings.
All in all, as soulless being, villains that either lost their soul, lack of soul in the first place, or naturally soulless potentially can/doomed to become a complete monster depend of the plot of story/fiction they appear in. This was due to main symptom of lacking a soul render them incapable/unable of any humane emotion at all, include empathy or regret over their actions. Soulless villains also became vulnerable with possession or evil influence from other villains. Even if not possessed/brainwashed nor used by other villains, their unnatural nature made them potentially became threat to any other creatures with souls if they did, no matter whether they are heroes or villains.
- Angels in Supernatural have been confirmed to naturally lack souls due to their nature as spiritual beings, unless they lose their grace and are rendered human.
- Blackheart from Marvel comics.
- Centurious from Marvel Comics.
- Count Abyss from Marvel Comics.
- The Dementors have been confirmed by J.K. Rowling to be soulless creatures.
- Elaine Heyworth from Shadow Hearts, an example of Soulless Shell that became evil due to controlled by demonic forces that originally used to revive her.
- Fabrication Machine from 9.
- Grims from RWBY, example of villains that naturally soulless.
- Homunculi from Full Metal Alchemist.
- The Heartless from Kingdom Hearts series.
- Hunson Abadeer from Adventure Time.
- Most of the man-made transformers of KSI are soulless, with Galvatron as the exception for he was in fact the new body of Megatron. Later, fifty KSI transformers were controlled by Galvatron and became Decepicons.
- The Leviathans in Supernatural are presumably soulless, as they are said to be beings which predate the soul.
- Lord Voldemort from Harry Potter series, partially. This is due tho the fact that his soul (which still in his body) has already torn apart since he creating Horcruxes. Also, while technically not the same as soullessness, Voldemort was born without the fundamental ability to love due to being conceived under a magic-induced fake love.
- Michael Myers (Halloween) from Halloween, as result of Curse of Thorn that has placed on him prior to his first murder, though whether his soul indeed still remain in his body or not is unclear.
- Ryan Anderson from Angel is confirmed as a human who was born without a soul, and is feared even by demons because of it.
- Shadow Detective from Yobi the Five Tailed Fox.
- Stalker from DC Comics.
- Soulless Sam Winchester, and the soulless victims of Abaddon's soul-mining operations, in Supernatural.
- Vampires in Romanian/Transylvanian folklore.
- Vampires and other demons from Buffy the Vampire Slayer.