Unique Physiology, Increased
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). In the anime, they are the product of a normally pure being consuming a pure innocent human soul. 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.
When a human commits a crime, they risk becoming a Evil Human. Depending on the severity of the crime, the individual actions, and reason, they may become an Evil Human. Their souls are normally indistinguishable from that of an human and Death is the only person who is able to judge the purity or impurity of their souls. If he deems them evil, he normally adds them to a List for Shibusen Demon Weapons to execute.
If a Weapon should start to consume a human soul that is not on 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.
A Kishin Egg is the soul of an evil human. It became evil by consuming other human souls, and thereby losing it's humanity and turns into a Kishin Egg. If the evil human is left alone to eat more human souls, it has a risk of becoming a powerful Kishin. Kishin Egg souls are deep red with hexagonal plates attached to the outside.
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.
This is, however, somewhat considered a inconsistency. This is because Death explained in the anime that only Demon Weapon can become become a Kishin and consume souls much like the Manga did. Many of the Evil Humans who consumed Souls in the Anime didn't do so in the manga and were hunted because of crimes such as murder and no soul consumption took place.
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.
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.
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).
Relationship with Death ScythesFor 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.
Many Evil Humans seem to display the same abilities that of a human or a Demon Weapon has though there's some differences such as:
Because of their evil deeds and crimes, they risk transforming themsleves into something more inhuman. Some Evil Human retain their human appearance while others transform drastically. Some gain extra abilities from their transformation while others may stay the same.
Due to the purity and power of a human soul, an Evil Human's power is normally increased more then that of a regular human.
Risk of Insanity
Due to the process of consuming human souls, any individual who is an evil human risk losing their sanity. Any Demon Weapon(Or Evil Human in the Anime) that eats a human soul risk losing their sanity at the expense of a power increase.