Here's my theory. I think Amon is someone new, however, since Korra's visions always appears when Amon does, I believe he is related to the past by being the victim of Yakone (the guy in Korra's visions). Yakone is the firebender who killed his parents and scarred his face. The reason he hates the Avatar is because Aang failed to stop Yakone from doing this because he held back, as was Aang's weakness in ATLAb. For how Amon acquired his ability to take away bending, my guess is that either Aang taught him through guilt, Amon learned it from seeing Aang performing it (because he's really smart), or it was an accident if Aang used it on Yakone and then Amon jumped in and somehow absorbed the power from Aang.
Amon can't be Tarrlok because his voice is far too different and his skin isn't dark as we can see from his hands. I think they chose that voice for Amon because they don't intend to ever unmask him, like how Slade was in Teen Titans. (I think his face is actually burned.) They might reveal his past and show what he looked like as a kid though.
Feedback would be nice. I also like hearing your theories, so tell me about them and I'll tell you what I think about them.