I was asked to give my opinion as to why I don't think it's Garth. Fine.
I don't think it's Garth for the same reason why I don't think it's Gary Mitchell, Roger Corby, Ron Tracey, Harry Mudd, or any other TOS baddie that you can come up with. The target viewing audience simply would not care about this. It's purpose would only be to satisfy trufan fanwankery, and that's not necessary anymore.
I've heard the rumors too that BC is supposed to be a TOS character. I personally think that's bullshit (or at the least, he'll be a TOS character in name only). The Dark Knight Rises is a perfect example of this. While Bane was a good villain in that movie, he was completely, completely different than the Bane from the comic. But Bane was chosen for a reason: he broke Batman's back in the comic, and he broke Batman's back in the film.
So with that logic, who in Star Trek's past wanted bloodthirsty revenge against Kirk? Garth? No. Gary Mitchell? No. Harry Mudd? No.
Except in this point in time in the AltVerse, Khan doesn't yet want "bloodthirsty revenge" on Kirk. And it doesn't even seem like the villain wants revenge on Kirk individually but on Earth and the Federation/Starfleet as a whole.
And the audience could easily care about Garth as a villain as long as the writers and Cumberbatch do their jobs and make them care
. You don't need to have prior knowledge of the villain to think he's bad news for the good guy. In that case, every Bond movie would suck because there's a new bad guy that nobody knows about in every film, and any movie with an original villain would fail as well.
To me, Garth has the best background to have his motivation easily explained in the narrative of the movie without having to do a heavy handed exposition. The audience won't have had to see a single episode of TOS to get that he is who he is, and why.
My Name Is Legion wrote:
It's not Garth.