This may be dangerous...but I have been pondering for some time the relationship and influence Moriarty and Sherlock have on each other as antagonist and protagonist. Having read the article, and others relating to what makes a good villain, on the blog referenced by Carl Powers in the Benedict Cumberbatch thread I have distilled some ideas as follows:

1. It would appear that the 'careers' of both Sherlock and Moriarty started with the murder of Carl Powers. Moriarty, either directly or indirectly responsible for the murder and Sherlock being the oly one realising the importance of the missing shoes and alerting the police. This is the start of their careers as 'consulting criminal' and 'consulting detective', even though they were both very young at the time. (TGG)

2. Both Sherlock and Moriarty have experienced ridicule. Moriarty gives as his sole reason for killing Carl the fact that he laughed at him - we never find out why. Reference also Mycroft's ridicule of John in TRF when John suggests that the person who divulged so much to the journalists might be an old school friend. Others on the forum have suggested this is Mycroft being cruel, in my opinion it is him being truthful, albeit bluntly. When Moriarty gives this as a reason for killing Carl, Sherlock would be able to acknowledge the effect ridicule may have on someone. Remember he was also 'hated' at University (TBB). They are both, to a certain extent, social misfits.

3. It is possible that Moriarty was at the pool when Carl died, however I think it is more likely that he got someone else to do the deed or at least to take the shoes. This would also fit in with his later comments that he doesn't get his own hands dirty, he usually gets someone else to do his dirty work. Sherlock too, at times, does not do his own dirty work either such as in SiB when he gets John to go traipsing off doing the legwork. ( The thing I will also say here is that, of course, the shoes are a plot device, however Moriarty all those years before would not have known that anyone would have followed up their disappearance, certainly not a young Sherlock, of whose existence he knew nothing. Why did Moriarty keep the shoes? )

4. When Moriarty shouts out 'That's what people do!' (TGG) this is echoed in Sherlock's speech to John on the roof of St. Barts, just before he jumps (TRF) where he also says, 'That's what people do, don't they?' There is a sense of a back story to this declaration by Moriarty and there are also hints of family loss in Sherlock's back story. Mummy is spoken of in the past tense 'You know how it always upset Mummy.' (ASiP) and no mention of a father figure at all. Sherlock snaps at Mycroft when he says, 'I'll be Mother,' when pouring the tea in the palace scene, with the retort, ' And there's a whole childhood in a nutshell.'

5. In the rooftop scene in TRF Moriarty says to Sherlock, 'No, you're me. You're me!' straight after Sherlock has said that he may be on the side of the Angels but Moriarty should not make the mistake of thinking that he is one of them. It is clear that it is quite possible, nay probable, that Sherlock could have gone exactly the same way as Moriarty. This links to the reference again of 'consulting criminal' 'consulting detective'; it takes a thief to catch a thief, as they say. Sherlock declares that he is prepared to do anything, to do things that ordinary people will not do and at first Moriarty is not convinced, however the 'Angels' speech convinces him and he realises then that Sherlock is actually just like him and really will do anything that is necessary. He realises that Sherlock is a mirror of him and visa-versa. It is surely no coincidence that antagonist and protagonist are standing so close, face-to-face in this scene, just like someone looking at their own reflection in a mirror.

6. In the scene in the flat (TRF), when Moriarty and Sherlock are drinking tea and verbally jousting with one another, Moriarty is literally a mirror-image of Sherlock. By holding his tea cup left-handed he mirrors exactly Sherlock holding his cup.

Finally to refer to a couple more ideas in the blog mentioned at the start of this post:

'A great villain UNDERSTANDS (the protagonist) ...better than any other living soul. That is the ultimate seductiveness of the devil - that he GETS you- right down to your vey soul. Moriarty seems to do this to Sherlock.'

'A story is very often a thematic argument between a hero/ine and an antagonist...On a basic level, the hero/ine represents one vision of how to live, and the antagonist another. Very often the antagonist also presents a dark vision of what the hero/ine could become...and it's through battle with the antagonist that the hero/ine is able to change.'

Food for thought...what do you think?