The last of my morality scale lists is the worst of the worst, the Complete Monster. These are villains that are pure, irredeemably evil and more often than not you really want to see them die. Because this can be a fairly controversial list, let me go out of my way to list things that will save a villain from this and things that will NOT save a villain from this. Let's start with the latter:
"But he loves his mother"-For me, playing this card doesn't work. A villain loving someone will not save them from this if they are sufficiently evil and I do not see consistent, genuine love. Merely sending a nice letter to a family member doesn't cut it. Nor is the psychopath who takes ten seconds to fondly remember his mother before going back to torturing and killing saved from this.
Faux-sympathetic backstory-He didn't finish art school. Someone said a mean thing to him. He had self-esteem issues. These things are not a sympathetic backstory and therefore a villain who becomes evil as a result of things like the above is just a petty individual who gets no sympathy.
Half-assed moment of decency-No, being a vegetarian when you're a mass murderer does not save you from Complete Monster status, nor does being nice to your waiter or secretary.
But he didn't start out evil...-This does not mean that he can't become a Complete Monster later.
Now for things that DO save a villain from this:
Being an Anti-Villain-The whole point of the Anti-Villain is that it is a villain who is NOT this for one reason or another. See those lists for the different kinds of Anti-Villains.
Consistent genuine love shown to another person-More than just a token mention of "but he loves his mother" or sending a nice letter to your sister, this is consistent signs that the villain loves someone and actually does nice things to them and doesn’t want to see them hurt. Cheshire and Black Manta from Young Justice are good examples of this.
Pet the Dog moment-If a villain does something genuinely good or heroic (like say, sacrificing themselves to stop a greater evil or showing genuine concern for another human being) than they are not this.
Affably Evil-These are villains who will only kill or try to kill people that are their enemies and/or are in their way, and to anybody else they're a perfectly nice person. Needless to say, this is not compatible with Complete Monster.
Failing the Heinous Test-So you have a villain who is all bad with no redeeming qualities. Surely that indicates a Complete Monster right? Not necessarily. Being a jerk or a scumbag is not enough to qualify as a Complete Monster. For a villain to be this, they have to be sufficiently heinous in relation to their peers. Also note that trying to kill the hero or heroes is par for the course if you're a villain and therefore is not enough to make a villain be this.
Redemption-Obviously if a villain pulls this then chances are they're not a Complete Monster since the whole point of those villains is that they're irredeemable.
Regret-Not necessarily redemption, but if a villain is shown to regret their actions, then they are not this. Regret implies a conscience, which Complete Monsters don't have. That said, some villains who seem to show this but are pretty dang bad (such as Ultimate Red Skull and Comedian) may just be faking, and I take that into consideration.
Comedy Villains-Villains who come from a comedy cannot qualify as a Complete Monster. The reason for this is because comedies are not meant to be taken seriously and Complete Monsters must be taken seriously at least most of the time. So you can see how the two are not compatible.
Something that MAY save a villain from this depending:
"He can't choose to not be evil"-With this it depends. If it's "he can't choose to not be evil because he was forced to become evil against his will" then yeah, the villain in question is not this due to their being forced to be evil counting as a sympathetic backstory. If, however it's because they are literally "made" of evil, than no, I don't think that saves them because I could argue that their being a Complete Monster is being invoked by their being made of evil.
And that's everything. So without further adieu, I give you the list, now updated with more examples and divided into two parts due to the large number of examples. This part includes the examples from DC and Marvel comics. Feel free to add names if you have suggestions, but remember all of the above before putting forth names. Also be wary of spoilers to various stories.