They are Humans with "corrupted" souls. Once on the loose, Evil Humans are a threat to Humanity in their quest for power by killing and collecting their souls. If enough innocent souls are collected, Evil Humans can become the very powerful Kishin; a being capable of submerging the world in fear and madness.
Therefore, one of the main objectives of the Death Weapon Meister Academy is to kill and dispose of Evil Humans as well as collect their souls in order to protect peace in the world and prevent a new Kishin from rising.
When a Human commits a crime, they risk becoming an Evil Human. Depending on what crime, the individual actions, and reason, they may transform into an Evil Human. Their souls are normally indistigushable from Humans and Lord Death is the only person who can judge the purity and 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 run the risk of going insane.
Abilities and Weaknesses
Because of their evil deeds and crimes, they risk transforming themselves into something more inhuman. Some Evil Human retain their Human appearances while others transform drastically. Some gain extra abilities from their transformation while others 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 that eats Human souls risk losing their sanity at the expense of increased power.