Oh dear, Oliver Twist-not my favourite of books. Anyway, my take on the question would be as follows; They are bad because they are directed that way by the social status quo either conditioned by their position in society (Noah and to a lesser extent Nancy,) by what society demands of them (Bumble, who genuinely believes he is looking after the workhouse children in a proper Christian manner because society dictates that anyone from their background is a born reprobate), and in only one or two cases do we actually get genuinely bad individuals (Sikes obviously springs to mind.) Dickens' characters are more an accusation directed towards society than humanity. Their secondary purpose is to accentuate Oliver's own angelic nature, as he comes out through all the hardships he has had to bear without the slightest taint on his character. In other words, the aim of the novel is to portray something that W.E. Henley managed, in 'Invictus,' to demonstrate far more eloquently and in just 16 lines!