Fagin is the first real antagonist that we meet, so we’re listing him first. He basically holds Oliver prisoner while training him up to be a pickpocket, and shows what he’s capable of when he loses his temper. The way he has an army of boy pickpockets and young prostitutes under his control is pretty creepy – it’s like a diabolical version of the way a mentor guides his young followers.
Monks is the shadowy mover and shaker all throughout the novel. As it turns out, he’s the primary person responsible for everything bad that’s happened to Oliver, right from the beginning. But since he’s not introduced until midway through the novel, and he’s not really identified until close to the end, we’re listing him after Fagin.
We’re listing "Victorian Society" here because that was Dickens’s real target – the system that was designed to take care of poor kids actually helped to corrupt them and turn them into criminals, and Victorian society was too callous and self-centered to care or to notice.
Bill Sikes may not have it in for Oliver the way Fagin and Monks do, but he’s awfully scary, and he’s the only one in the book who actually commits a murder. So even though he’s not an antagonist on the same level as Monks, Fagin, and Victorian Society, we still think he should be included here.