No, he's not a demon. He's just a perfect example of how demonic a human can be.
And as to why I know so much? Mr Holmes, from what I can tell, has just encountered him for the first time. I'm from a lot further in his future, and I've encountered him more.
[Ruby]
I assume Sherlock thinks I'm his spy? You don't have to confirm; but Moriarty has cleverly made it look like I'm working with him (to Sherlock at least) and I would guess Sherlock is espousing his usual disbelief in coincidence and blaming me.
You (and by you I mean all the people here) have been very kind to me and I wouldn't belittle that by working against you. Please believe me on that.