Okay the Biology in me is going to come out. I am not an English person so this is my perspective. Coming from a biological standpoint I would say that genetics might have a lot to do with why Oliver is not a bad person. It is the argument of Nature vs. Nuture. I know that Oliver was not put into a good environment, but maybe his genetic goodness was stronger.
Many psychologists say that the envirnoment plays a key role in the development of a person.
Maybe people like Sikes already had genetic tendencies to do evil and the environment brought them out even more. I also want to say that I don't really think Oliver knew he was being such a good person, it just seemed to come naturally to him. People like Hitler for example obviously have some sort of psychological problem. Fagin would also have some sort of problem that was brought out even more. I'm sure that Dickens problaby wasn't thinking in these terms, but I think in the matter of biology or psychology it can be seen this way.