I decided to try him but given the new tragedy preventions make me iffy here.
What's the work?
That Time I Got Reincarnated as a Slime is an light novel about an worker who was reincarnated as a slime who formed an nation and befriended a dragon while trying to make a nation to enjoy his life enjoyable like in his old world. It also got Anime, Manga, and mobile & console games too.
Who is he?
Vega is one of the antagonists who is originally ally of Yuuki Kagurazaka and later a member of Army of Justice who is a sadistic man-eater.
What Makes Him Close to Pure Evil?
Heinous Standards?
There is the huge high heinous standards where genocidal crimes a bit common as there is Guy Crimson, Milim Nava, Veldora Tempest and more wiped out entire nations or Mariabell Rosso and Yuuki Kagurazaka that tried to destory the Jura-Tempest Federation.
Although Vega has lower resources than most villains and plus other reasons.
Heinous Actions
He was shown to attempt on feeding humans before being captured and was originally gonna be disposed of before Yuuki Kagurazaka secretly smuggled him away for education.
In present day time in Eastern Empire Invasion he would feed on his own allies before deciding to aid the omnidical maniac Feldway in his schemes of destroying the Central World.
In Disturbance of the Royal Capital where in aiding the Army of Justice to attack Engrassia, he would use his skill to turn several people into Evil Dragon Beasts, one of his victims was shown to be put in a "I Scream"-like suffering fate meaning it can be applied to the rest of his victims and intended on unleashing them in the city captial if not for the heroes driving him away.. He also had intentions of making Hinata Sakagushi his plaything in one instance.
He would later try to attack the Dungeon later on but would be banished from the Central World, basically imprisoned in dimensions played for satisfaction.
Possible Preventing Factors?
He isn’t loyal to anyone at all given he will switch sides when time turn on it even Feldway himself although Vega only made sure not to step out of line given he tried to ensure not to provide his wrath.
Despite some of his crimes being under orders of Feldway and Michael he wouldn't fail the heinous standards to him given he did it out of pleasure and sadism plus turning those into Evil Dragon Beasts shown to be a "I Scream"-like fate is unique of a crime to stand out.
What Prevents Him From Being Pure Evil?
While it mostly don't hold up anymore but he has a genuine enough of a tragic backstory to prevent Pure Evil status as he was shunned by everyone in his village including his own mother while trying to kill him over his connection with his father who is infamous for experiments forcing him to survive on scraps to survive making him learn of how harsh the world is and it isn’t played for too much sympathy due to it not being explored too much.
It isn’t like where he was abused, sexually assaulted, or put in a "I Scream Fate" like new criteria. Just only just shunned from others as a child and forced to search for scraps to survive and he wasn’t mentally traumatized by it or severely badly effected by it given he seemly mostly fine and not badly effected but it enough to prevent PE status given it effects him enough that it did show him not to trust others or how harsh the world is to survive so it still hold up enough for Minor Tragic and isn't too badly effect it to be IH either given he acts mostly fine here.
Plus others like Shizue Izawa and Chloe Aubert been through just as bad if not worse yet turned out to be good people.
Also given he did aid a omnidical Feldway, thought of making Hinata his plaything and put several in a "I Scream Fate" through making them several Dragon Beasts make it hard to sympathize too much.
Verdict
Up to you all to decide