It is Sebastien Moran, and I actually like the way they played him as being a dangerous professional killer who still has enough cunning to realize when he's been set up by his employer. I'm wondering if they're going to keep Jones on the show and have Holmes and possibly Watson interview him for information on Moriarty himself.
Speaking of the Professor, I'm likeing the way they're building him up as a different animal from Sherlock's take on the character. While Sherlock featured the competitive "Consulting Criminal" aspect Moriarty displayed in Valley of Fear, while this show seems to be hinting at a Moriarty more in line with the ruthless yet profssional "Criminal Mastermind" introduced in Final Problem. And I hope whoever they cast resembles Jared Harris's take on the character more than the Sherlock version, just because Sherlock's version seemed a little too unstable to me. He was fun to watch, he was just more of crazed genius than the coldly calculating shadow from Doyle's books. Plus, I really appreciate a charmingly chilly bad-guy like Harris; he was pretty clearly interested in Holmes as a peer and enjoyed their games, but was still detached enough to need the personal prodding of Holmes to take the fight personal.
And I loved Moriarty's plot in this episode. We find out that Moriarty had Irene killed (still thinking she helped him fake her death and we'll see her before we see Moriarty himself) almost assuredly to throw Holmes off his tracks. He then sends Moran on a collison course with Holmes in Holmes' new turf, setting up either one to destroy the other, with a double victory if Holmes cleans up Moran as a loose end and is arrested for cold-blooded murder. It's quite brilliant when you think about it, with only one flaw getting in the way:
Neither Moran nor Holmes are the statistically predictable "average man." So Moran's use of his cunning to figure out that he's been set up and anger at the betrayal has him reveal Moriarty's name early and plot for revenge, and Holmes's cold, methodical plan for revenge gives Moran enough time to tell the truth. And even with this loss, all Holmes has is a name and a tricky witness who is probably looking for revenge himself.