In light of Meat Trademark's answer the following discussion about Moriarty's motivations might be moot, but I'll still stand by this answer as someone who has only seen the first two seasons yet and tries to give a valid explanation in light of those, disregarding any future revelations.
The main point is indeed his insanity. First of all, insanity can be a reason for pretty much anything, especially suicide. But to be more specific, Moriarty's ultimate goal was to win against Sherlock in a battle of wits. As Keen says in his answer, Sherlock reasons about the possibility for Moriarty to call back the assassins and thus for Sherlock to not have to kill himself. So as long as Moriarty lived, Sherlock wasn't broken yet. So Moriarty leaves him with (supposedly) no options left when he kills himself, thinking that he has finally defeated him mentally.
And in fact Moriarty's brilliance was his insanity, or to be exact, his judgement of his brilliance to be more important than his life. For him there was something "greater than life", it was to win against Sherlock Holmes. There are enough examples, be it in history or the movies, of people sacrificing themselves for (what they think to be) higher ideals that are more important than their own lives, and James Moriarty is one of those.
So the question is not if we would do the same or what we regard highest in the world, but if we can understand why he did so given his portrayal throughout the show, and for me this was a perfect ending for him, nobly sacrificing himself for the ultimate win (the biggest problem I would have with doing so was the inability to make sure that I was successful at all, which unfortunately for Moriarty, was not the case).
As to the updated question, Moriarty might have been very smart, but he also was extremely arrogant in his believes that noone, not even the great Sherlock Holmes could beat him in a battle of wits. When he killed himself he actually thought that he had considered (and prevented) each and every possibility for Sherlock to solve this final problem. In retrospecitive one could say it was a stupid idea to kill himself without being sure that Sherlock couldn't survive, but Moriarty really was 100% sure, blinded by his arrogance and superior intellect and his insane will (and confidence) to prevail over Sherlock.