As the main protagonist of R. Scott Bakker's The Second Apocalypse, there are solid arguments to be made for him being either Pure Evil, Near Pure Evil, or Inconsistently Heinous.
For those unfamiliar with TSA (which I assume is everyone on this sub), Kellhus is basically Aryan, god-king Jesus with psychic powers who conquers much of the setting to become strong enough to overthrow the gods.
The thing about him is that, as a Dûnyain (basically a biological supercomputer operating entirely on logic), he is completely devoid of empathy and humanity, being completely single-minded when pursuing his goals, regardless of the suffering and costs. Virtually all relationships he forms are transactional and ultimately one-sided. Because of this, Kellhus genuinely gives Fang Yuan a run for his money in how ruthless and cold-blooded he can be.
In fact, despite TSA being grimdark enough to make Warhammer 40K look like Fairy Tail, the things Kellhus does (from raping and mentally breaking his most devoted follower to committing genocide) make him stand out as one of the setting's biggest bastards.
The biggest potential redeeming quality he has is his extremism: the desire to save humanity from predetermined damnation. Since virtually all of his worst atrocities were committed in service of this goal, it could be reasoned that Kellhus is a shoo-in for Inconsistently Heinous.
But from what I understand, Near Pure Evils can be extremists as long as there is some ambiguity to their motives or the narrative downplays their positive intentions. Pure Evil characters who claim to be extremists (like Funny Valentine) can have their intentions called into question, if not outright subverted, by the narrative via factors like pride or hypocrisy.
Unlike fellow literary god-emperors extremists, like Leto II or the Emperor of Mankind, Kellhus strikes me as the kind of guy who wants to save humanity purely for the sake of it. For someone completely devoid of compassion and empathy, having humanity seems more like an obligation than altruism. As a Dûnyain, who seek to attain absolute spiritual autonomy by overcoming all obstacles, what is escaping damnation other than the biggest obstacle?
While he definitely qualifies for Inconsistently Admirable/Conflicting Good, I have found there to be so much ambiguity and philosophical nuance regarding Kellhus' true nature (even after seven novels) that I still can't make an educated guess on how evil he is. Truly, I have not encountered a character like him.