So seeing how we discussed least favorite protagonists in fiction, how about we follow up with this question:
Who is the least likable main antagonist in fiction you've ever seen?
My pick would have to be Biba Amatori from Kabaneri of the Iron Fortress. The moment this character was introduced, the show went downhill for me. He is easily one of the most despicable characters I've ever seen in fiction. Biba's main motivation is to kill his own father, the emperor, just because he left him behind during a battle. Thus, Biba plans on getting revenge on him by turning him into a Kabane.
He succeeds, but then he takes things a step further and decides to unleash the Kabane virus on the entire capital. Um... excuse me? Biba, you got what you wanted, and now you want to cause the apocalypse? That makes zero sense! He also unleashed the Kabane virus on another city prior to what he did on his father, ending in that city's annihilation. His motivations for why he wants to unleash the virus make zero sense. He goes on how he wants to "test the strong," but there is never a proper explanation behind why he thinks that way, which makes him come across as not only irredeemable, but also terribly written.
What's worse is that the main heroine of the show, who has a past with Biba, still shows sympathy toward him even after killing him and gives him a proper burial, when in reality, what he does throughout the second half of the show made him completely undeserving of that.
I'd also like to give a dishonorable mentions to two characters: Sugou Nobuyuki from Sword Art Online and Zouken Matou from the Fate series.
Sugou is both absolutely despicable for being an over-the-top molester toward Asuna, who he keeps caged and plans on forcefully marrying against her will, and he is poorly written for being a mustache-twirler who does evil things just for the sake of being evil with no depth behind his actions.
Zouken Matou is an irredeemable monster throughout Fate/Zero and Fate/Stay Night Heaven's Feel. In Fate/Zero, he admits that he thoroughly enjoys making his own son, Kariya Matou, suffer. This is shown shortly after Kariya is implanted with the worms, where he pounds his cane onto Kariya's uncle while laughing out of amusement as Kariya screams out in pain. He also laughs out of amusement as he forcefully feeds Kariya a giant worm around the middle of the show, the latter once again crying out in pain from the torture. Zouken is also responsible for turning Shinji Matou into the despicable kid that he is in Fate/Stay Night, basically hammering down Shinji for his lack of magical talent which in turn led the latter to become more cruel in his actions towards others.
