In Soul Eater, Evil humans are humans with "corrupted" souls. In the manga, they are indistinguishable from the souls of a normal human, but their souls have become tainted by evil deeds (usually murder). Only Lord Death can judge the quality of a human soul, and he has a list of individuals whom Meisters may kill and feed the Souls to their Weapons.
If a Weapon should start to consume a human soul that is not on Lord Death's list, that Weapon's soul becomes corrupted and they become a Dark Weapon. Consuming pure souls is the one way to become a Kishin, and these Dark Weapons are in danger of going insane.
In the anime, they are the product of a normally pure being consuming a pure innocent human soul. The end product is that the human's soul becomes corrupted and gains the urge to feed on more souls. Ultimately, this leads to them being hunted down by Meisters and killed before they become a Kishin. The anime shows evil human souls to be red in color with dark 'scales' on some parts of the surface once killed.
Normally, a human proceeds to consume another human's soul out of some sort of fear, a means of gaining power, or due to a personal reason that are unique to that person. The Star Clan, for example, is infamous for becoming obsessed with power and consuming innocent souls. Masamune, Tsubaki's older brother, devoured souls due to an inferiority complex developed after Tsubaki inherited the majority of their family's Weapon heritage. Once a pure human has devoured a pure soul, it is considered to be on the path of a Kishin. After consuming enough souls, the Evil human can become a Kishin.
Most Evil humans, when fully transformed, no longer resemble a perfect human form and take on strange distorted humanoid figures, like with Jack the Ripper, a long-limbed grey-skinned vaguely human-like creature with metal claws, and Rasputin who takes on the form of a giant human-like form. However, there have been exceptions, as Masamune looks perfectly human, despite being an Evil human, although during the fight with Tsubaki, his human form starts to display Evil human-like characteristics, especially in his facial features. Comparing Jack the Ripper with more human-like Evil humans such as Masamune and Alcapone, the appearances of Evil humans seem to vary. Strength and power and personality also seems to vary, and may very well depend on what sort of character the Evil human was before walking the path of the Kishin.
Once a human becomes an Evil human, their soul turns into what is called a Kishin's Egg. Since the aim of all Evil humans are the same (to kill and devour souls) their souls all take on the same appearance, that of a Kishin Egg. A Kishin Egg is a type of soul that is red with a purple core and covered with dark brown 'scales' on some parts of their surface. Although the appearance is the same, the size of Kishin Eggs seem to depend on the level of strength of the Evil human (the stronger the Evil human, the bigger the Kishin Egg).
Evil humans also leave behind no corpse when slain (a fact ignored in the first few chapters) often disappearing in a wisp of black smoke, leaving behind only their soul. Almost all Evil humans also share a deep hatred for Shibusen, mainly because of their campaign against their activities.
The Evil humans also gain a new name, mostly named after famous or legendary characters in history, with Alcapone, Jack the Ripper and Rasputin already being examples. However, given the nature of the world and how many characters throughout the series are references to things in other media, these may be their actual names. Meisters when approaching an Evil human with their Weapon when about to battle it, usually address them with this name. The Evil humans also sometimes possess a title, with Jack the Ripper being 'Serial Killer' and Lupin as 'Master Thief/Phantom Theif.'
The Evil humans also possess different levels of intelligence. Whilst some like Jack the Ripper are almost animal-like and driven to a mindless frenzy for power, some like Alcapone are devious and cunning, gathering associates to increase their power. Overall, all Evil humans seem to be capable of at least a small amount of speech.
However, all Evil humans share the same goal: to consume lots of human souls in order to become more powerful. This results in their homicidal and antagonistic behavior in order to consume innocent human souls (or even a Witch Soul in Alcapone's case). Also, most Evil humans seem to be nocturnal, only coming out at night to hunt for souls, although there are exceptions.
The DWMA's Duty to Prevent the Rise of a New Kishin
Once on the loose, Evil humans are a threat to humanity in their quest for power. As well as the amount of innocent people that will end up killed in an Evil human's path, if the Evil human is left to thrive, it would transform into a very powerful Kishin, a being capable of submerging the world in fear and madness.
Therefore, the disposal of Evil humans is one of the main objectives of the Death Weapon Meister Academy in order to protect peace in the world and prevent a new Kishin from rising. The Academy trains Meisters, humans with powerful souls and Soul Wavelengths, with a Demon Weapon partner from a young age. The Meister and Weapon teams are sent out on assignments to kill Evil humans and obtain their souls in order to make their Demon Weapons stronger.
Relationship with Death Scythes
For a Weapon to become a Death Scythe, they must defeat and eat 99 corrupt human souls and one Witch Soul. Despite the large amount of Evil human souls that are needed, this is often considered the easiest part of the job, as most Evil humans resort to basic melee attacks that are easy to counter for a trained Weapon and Meister team. The Witch Soul, however, is considered the most difficult and has taken the lives of many Meisters, mostly as the Witch can know powerful magic attacks, and are considered immortal as well.
They can be 'absorbed' in different ways; Liz and Patty have been seen absorbing them via their magazines. Soul has been seen eating them.