While the sort of thing that happens in Oliver Twist is based to some degree on fact, Dickens has always tended towards writing caricatures, which was in fact common with a lot of Victorian literature at the time. In a Dickens book, if a guy was bad, he was cartoonishly, vindictively, bad. If someone was good, they were saintly good.

Not every adult in those days was as evil as they seem in Oliver Twist (I mean the fact that Dickens, an adult, was writing about how horrible these conditions were suggests he was not alone in his opinion). But for the story he is telling, he had to make them extra horrible to get his message across.