Mithalwen and Osse brought up the role that genetics and upbringing might have in determining one's goodness/badness. Sadly this question is not solved in your world, let alone in Middle Earth. The 'super males' and 'super females' have a lower IQ, therfore a tendency to solve problems using physical violence and a short attention span. That makes them prone to relatively minor crimes like robbery and assault. A good upbringing will guarantee that desirable behaviours will be enacted
if as long as they are reinforced. In other words, I'll have nice table manners because daddy promised me a pony ride. That does not exclude the latent existance of undesirable behaviours that will rear their ugly head when opportunity knocks.
Alright, enough of that. After all, this is only a story and real life can be much more complicated. The bottom line is - if one can prove, based on Tolkien's own assumptions, that, if a kindly Elf had reared Shelon since she was an teeny weeny baby spider, she would not have become the 'Dark Goddess of gluttony', than the question's settled: she's not a monster, the true monster is society.
Same with orcs.
As an afterthought, I always get the impression, based on absolutely no cannonical sources at all, that Tolkien was feeling more kindly towards orcs than he let show, and similarly, more pessimistic towards men.