Complete Monsters (also known as Devils in Person and Pure Evils) are the most evil and worst type of villains, the wrongdoers who are completely evil. And for this kind of villain, doing evil for them is as natural as breathing. These evildoers must have absolutely no redeeming qualities, commit atrocious actions and show no regret for their crimes, and commit actions that are extremely unforgivable. Most, but not all monsters cross Moral Event Horizon as they do atrocious acts. If these primary monsters do not die or get killed by protagonists in the end of the story or novel or series, it can lead to sad or bad ending. However, some of these monsters can lead to downfall as Damned Souls after they die or are defeated by heroes. They are the direct opposite of Pure of Heart as heroes are incorruptible.
This type of villain is the last step from any imaginable bound. One that is beyond evil and wicked. They are truly irredeemable, as Complete Monsters are that type of villain that you cannot can reason, instead you need to stay away from them or simply destroy them for good.
Many of these villains fits in category called Masters of Horrors due to they are free from compassion, love and any shred of humanity. But a Complete Monster is not necessarily a Master of Horrors, as Master of Horrors in general are far more horrifying than typical Complete Monsters. In fact, some fictions that featured both a Complete Monster and a Masters of Horrors often shows that Masters of Horrors usually pose greater threat than standard Complete Monsters that when comparing his deep they crossed MEH, Masters of Horrors usually beats Complete Monsters in terms of how deep they crossed.
But regardless, in terms of whether they had redeeming qualities or not, true Complete Monsters usually irredeemable. Some fictions that featured Complete Monsters in their stories may tested these characters whether they still had a shred of redeeming qualities or not by creating circumstances where these Complete Monsters brainwashed/possessed to the good side with side effect in form of them losing memories of their past as evil person (e.g. Darth Reva, Professor Pericles, Darkroom). The true test began when they finally discovered their past identity as villains by uncovering their lost memories or belongings that they possesses when still evil, or both. Only true Complete Monsters that immediately returned to the dark path on a whim while the ones that had redeeming qualities would greatly depressed with their dark past and would try the best to redeeming themselves (more than often that these people had to use new identity for sake of people they used to antagonized as using their old identity will brought bad memories of their past self).
Some of this characters may started as either good guys, heroes, or minor characters whom ultimately turned to the dark side and ultimately went to the path that they can never return.
Requirements for a villain to be a Complete Monster:
They commit acts that are atrocious by the standards of the story.
They are taken seriously, causing fear, disgust, and hatred from other characters in the story.
They are hopelessly beyond redemption and never show any regret for their actions.
Anti-Villains can never qualify seeing as to how you are meant to sympathize with an anti-villain, even if you do not approve of their methods. Also, just being a obviously evil character such as a serial killer does not automatically make the villain a Complete Monster unless they are particularly brutal in nature.
I met him fifteen years ago. I was told there was nothing left. No reason, no conscience, no understanding; even the most rudimentary sense of life or death, good or evil, right or wrong. I met this six-year-old child, with this blank, pale, emotionless face, and the blackest eyes... the Devil's eyes. I spent eight years trying to reach him, and then another seven trying to keep him locked up because I realized that what was living behind that boy's eyes was purely and simply... evil.
You know you are involved with a sick man who will see you die? He will stand over your body, with your blood on his hands and I promise you he will laugh... not because your life means nothing to him -- but because death, for him -- is the punch line.