Spoiler for like, any book existing.
I think mine has to be the villain in Mirror Man. Idk, he wasn’t intimidating, he wasn’t interesting. He was meh.
Also the villains from The Chain. Dear God they weren’t sympathetic in the slightest. The book tries to act like they were atleast pitiful but FROM THEIR INCEPTION, they were just evil assholes who enjoyed causing pain. The book itself wasn’t all that great anyways.
Difficult to be worse than many of the characters mentioned so far, but I’ll go with Francis Begbie from Trainspotting novels as he hasn’t been mentioned yet. He’s a psychopath prone to spontaneous violence and manipulating his ‘friends’ to his will. Ghastly fellow.