These aren't even boss characters as those are a different breed of evil altogether. Some of the most evil characters in Elden Ring are mere NPCs who are on the same level as the Tarnished players. Arguably, that makes their existence more chilling since they can't use the "demi-god" excuse of why they're terrible. Don't feel sorry for killing these NPCs.
Poopy Nomnoms over here needs no introduction, the game's opening already explained his personality well. But if it wasn't clear already just how malevolent Dung Eater is, he's possibly the worst serial killer in the history of video games. His signature killing method is consistent; all his victims have blood dripping down from their inner thighs.
It's up to the players' imagination what exactly his "defilement" entails. Even without those heinous acts, Dung Eater's goal of cursing everyone to become an Omen or something is already a horrific ending for the game. He's the perfect embodiment of chaotic evil.
She doesn't need to declare that she eats dung to be considered a creepy character. Lady Tanith of Volcano Manor prefers a different kind of cuisine; her husband mainly, Lord Rykard. If players defeat Rykard, talk to Lady Tanith, and then come back to Rykard's boss room, they'll spot Tanith in action.
She'll be consuming Rykard's disembodied head in an attempt to become a part of the Serpent family and as a way to be with Rykard forever. Other transgressions include inducing amnesia in her foster daughter by poisoning her and asking players to carry out senseless assassinations to spite the Erdtree.
Seluvis will do since this creep doesn't deserve his esteemed title. He's a brilliant sorcerer alright, except he uses his mind to concoct a perverse plan of turning people into puppets. He has a hidden man cave in the Three Sisters area where he keeps all his flesh puppets and these things used to have their own will and are their own person.
His hubris also knows no bounds as he even planned to betray Ranni and also turn her into his puppet. That's why he's been hanging out with her under the guise of a servant. Players don't even need to know all that to hate Seluvis. His general dialogue and penchant to belittle and insult anyone that isn't a demi-god are provocation enough.
Turns out Seluvis isn't alone in his endeavor of turning living beings into playthings. Pidia, another sorcerer hiding in Caria Manor, is apparently best pals with Seluvis; the two of them likely help each other in procuring puppets and improving the methodology.
Those brainwashed soldiers in Caria Manor also seem to belong to Pidia as they do his bidding and will. It's also implied during his death dialogue that he practices something "blissful" with all his puppets- without their consent, of course. Thankfully, the puppets got their revenge. It's a shame Seluvis' puppets were denied that satisfaction.
Sellen, despite distancing herself from Raya Lucaria Academy, is still a questionable figure in Elden Ring. She's aware of her crimes, of course and even acknowledges the fact that she's a witch who got kicked out of the academy for her unspeakable practices. But players can get a rough idea of what Sellen thanks to her questline.
She's not that far off from the likes of Seluvis; at one point, she even asked for Seluvis' help. A kind and sane person wouldn't do that. Sellen probably asked for a fresh puppet body (or bodies) from Seluvis, so she can transfer her consciousness and have multiple selves. These bodies are not magically made from thin air in Elden Ring, meaning these three sorcerers (Seluvis, Pidia, and Sellen) are on the same level as Godrick the Grafted when it comes to mass sacrifice.
It's easy to mistake Shabriri for Yura once players meet that ronin attire in the Zamor Ruins Site of Grace. But since Yura died in the Altus Plateau, it's another person who took over his dead body. That would be Shabriri who may or may not be the manifestation of the Frenzied Flame since all he does is spew madness and nudge players into serving the Three Fingers.
He's certainly a shady individual who has a penchant for gouging out eyeballs as some kind of ritual and sustenance for Finger Maidens who wish to serve the Frenzied Flame. Regardless, anyone who serves the Frenzied Flame is up to no good since pursuing that entity will result in a mass genocide ending for Elden Ring.
Surprised to see this guy? He helped players sneak past the main gate in Stormveil Castle; how can he be evil or creepy? Because he steals Runes from the players every time they die. Gostoc is also the culprit who locked the players in a dark room with a Banished Knight in Stormveil Castle. One can even hear and recall his laughter. He's no better than Patches, perhaps even worse.
Gostoc has a habit of looting from corpses to fund his shop and business. It just so happens that players typically die loaded with Runes, all the more convenient for Gostoc. So just because some NPC helped the players with a secret path doesn't mean they have good intentions.
White Mask Varre is the first NPC with whom the players will speak just outside the tutorial area and that early, he's already reeking with a seedy aura. Progress a bit into Liurnia of the Lakes and Varre will reveal his true blood-red colors. He's actually serving Mohg, the Lord of Blood, who's a vassal of the Formless Mother, an Outer God and enemy of the Erdtree.
These folks are a sordid bunch as Varre will even ask players to disrespectfully invade others to slaughter them in an ambush. Meanwhile, his boss, Mohg has a proclivity for kidnapping an underage demi-god to corrupt the poor infant's blood. But above all else, Varre calls out the players for being maidenless; the best retort to that is by making him lifeless.
Elden Ring is available on PS5, PS4, Xbox Series X/S, Xbox One, and PC.