Moriarty never intended to kill Sherlock, just make him disappear in the most debasing way possible.
The running theme of the season is Sherlock's rise to fame, starting in Scandal and peaking in Reichenbach. Can you be an effective private detective if everyone knows who you are? Sherlock answers that question himself in a throwaway line in Scandal that could be conveniently summed as: "No". Just as importantly, from Moriarty's point of view, can you fight a shadow war with someone the press keeps the spotlight on? Hell no: that's bad for business and if your business is criminal you don't want to be the known enemy of someone playing centre stage of a media circus.
So, you'd want them off the board but... if that's the case... what's all the talk of an IOU about? Despite the malicious posture, Sherlock's never really stopped Moriarty from doing anything of note yet - he's been more of a thorn - so what's he owed outright for? Well... lets go back to Scandal: it's established in the opening and towards the end that Moriarty and Irene Adler have a working relationship of sorts. Given the outcome of that case, could Moriarty feel that he owes Sherlock a favour? The character's always been designed around a twisted morality when it comes to such things.
So, could he feel that he needs to help Sherlock regain his anonymity to clear a debt and let them get back to their private game? If so, how can he do so thoroughly and without looking like he's trying to do Holmes a favour? Destroy his reputation with extreme prejudice and then rig his death?
Hell, working out the mechanics of that while staying a step in front of Holmes might just be fun too...